Hrmm.... Let's see. Constitution damage/loss/reduction results in lowering not merely currenty HP by 1 per hit die, but also maximum HP by that amount. 8th-level cleric with 20 Constitution casts Blessing of Leeches. Assume average hit points from each level, but max for 1st, so 76 hit points. Round 1, heal 5hp to no effect, go down to 19 Con and 68 HP (68 max). Get hit by annoying giant for 21 damage, go down to 47 HP. Round 2, heal 5hp, go down to 18 Con and 52 HP (68 max). Get hit by same giant for 19 damage, go down to 33 HP. Round 3, heal 5hp, go down to 17 Con and 30 HP (60 max). Get hit by accursed giant for 12 damage, down to 18 HP. Round 4, heal 5hp, go down to 16 Con and 23 HP (60 max). Get hit by same giant for 14 damage, go down to 9 HP. Round 5, heal 5hp, go down to 15 Con and 1 HP (52 max). Become a gooey smear on the giant's club this round.
Alternative: Round 1, cleric casts an immediately useful spell like Shield of Faith, Doom, Protection from Evil, or whatever. Takes 21 damage, then 19 next round, then 12, then 14 damage, as above. By the start of round 5, the cleric has.....10 HP, instead of the 3 HP noted above. Just might survive the next attack, long enough for someone else to finish off the giant or force-feed a potion of Cure Serious Wounds to the cleric (or transport his unconscious, dying body to a safer spot first, then heal him). And unlike the above example, the cleric still can be healed easily to their usual maximum of 76 HP (Cure Critical Wounds, cast by himself or a fellow 8th-level cleric, should heal 24-40 damage per casting, minimum of 12 per casting). And the spell they cast on round 1 may have actually helped them avoid one or more of the aforementioned hits, by boosting their AC or reducing the giant's attack bonus.
Lesser example, more reasonable: 6th-level cleric, 16 Constitution, 46 HP. Round 1, attacks an orc with 5 warrior levels, firing crossbow. Orc moves in and hacks with greataxe for 12 damage (1d12, +3 Str for two hands, and for pity's sake we'll assume this particular orc is a complete idiot so didn't take the Power Attack feat to gain +10 damage for -5 to-hit), cleric drops to 34 HP. Round 2, cleric drops crossbow and steps back, draws his mace, and casts Blessing of Leeches, status becomes 15 Con and 39 HP (max 40). Orc hacks again, for 7 damage, drops cleric to 32 HP. Round 3, cleric strikes back and goes to 14 Con, 37 HP (max 40). Orc continues the stupid hackfest with a pathetic chop for 4 damage, cleric goes down to 33 HP. Round 4, cleric smacks the orc again and goes down to 13 Con, 32 HP (max 34). Orc slices into the cleric for 11 damage, dropping the cleric to 21 HP. Round 5, cleric strikes again and goes to 12 Con, 26 HP (max 34). Godawfully stupid orc tears into the cleric once more for 8 damage, reducing him to 18 HP. Round 6, cleric takes another swing at the orc, goes down to 11 Con and 17 HP (max 28). Orc strikes back, for 15 damage with a direct hit, knocking the cleric down to 2 HP. Round 7, cleric strikes one last time and goes to 10 Con, 8 HP (max 28). Orc renders cleric unconscious with one more axe strike.
Alternative: Cleric and orc bash away at eachother relentlessly, cleric doesn't cast any spells until the battle is nearly over. Cleric takes 12, 7, 4, 11, and 8 damage as above, then casts Cure Light Wounds instead of Blessing of Leeches, healing 9 damage, putting him at 13 HP before the orc hits for 15 damage, knocking him out on round 6 (whereas Blessing of Leeches would've saved him until the end of round 7). Advantage to BoL this time, barely, putting the cleric at 4 HP higher than Cure Light Wounds likely would. Of course, at 6th-level, the cleric has no way of healing all 6 Constitution damage he sustains before either falling unconscious and dying, or slaying the orc and dismissing the Blessing of Leeches. He may heal a few points with Lesser Restoration, but even after that, he and the party's druid or paladin or whatever can't heal him beyond 28 HP (34 after the LR). Not until he pulls off a few more Lesser Restorations later.
By comparison, if he or an ally simply cast Cure Moderate Wounds instead of CLW, it would've saved him from being unconscious on round 7, same as casting Blessing of Leeches, but without the hard-to-deal-with repercussions after battle. This would forego the need, compared to the BoL scenerio, to cast 2-3 Lesser Restorations and 1 Blessing of Leeches in order to achieve the same general effect. One 2nd-level spell (CMW) could do the job of BoL and 2 other 2nd-level spells (LR). Fewer resources wasted on just using a single CMW when the cleric's taken roughly ~20 damage.
One last, shorter example: 2nd-level cleric, 14 Constitution, 16 HP. Attacks 2nd-level orc warrior. Round 1, takes 5 damage, strikes back with mace, down to 14 Con and 11 HP. Round 2, takes 7 damage, casts Blessing of Leeches, down to 13 Con and 14 HP (14 max). Round 3, takes 8 damage, strikes back with mace, down to 12 Con and 11 HP (14 max). Round 4, takes 4 damage, strikes with mace, down to 11 Con and 10 HP (12 max). Round 5, takes 6 damage, strikes back, down to 10 Con and 9 HP (12 max). Round 6, takes 8 damage, falls unconscious before able to strike back, down to 9 Con and -1 HP (10 max). Alternative: Casting Cure Light Wounds once is not quite as effective. If the cleric gets no healing other than the single CLW, they will fall unconscious after the 4th hit, rather than the 6th hit. Although, with BoL, the cleric has to wait 5 days for their Constitution to recover, so they're boned if they get attacked again during that time (especially as their max HP will be 10 or 12, until their Con is mostly replenished).
By the by, Shield of Faith at 1st-level decreases the chance of being hit by effectively 10%, and by 8th-level it's a 20% lower chance of being hit (assuming the opponent's attack bonus isn't so high that it more or less never misses). This is less true at high levels since attack bonuses are far quicker in growth than AC bonuses, but nonetheless it's better to avoid a single 20+ damage hit than to simply heal that same amount over time, especially if you heal slower than the rate at which you are damaged. Even moreso when the opponent's hits inflict more than just damage, such as with a shadow's Strength damage, a wight's negative levels, or a wraith's Wisdom drain.
Also, take into account that Blessing of Leeches reduces not only Con and hit points, but also Fortitude and Concentration as well.
So Blessing of the Leech is mildly more effective and much more costly at low levels, but by the mid-low levels (5th or 6th at latest) it starts to outright suck and waste resources, until the cleric can cast Heal to both restore a bunch of hit points and cure all ability damage at once with a single spell, and even then it's wasting a 1st-level and 5th-level spell as opposed to expending 2-4 spells of 3rd or 4th-level (which, while perhaps a bit more numerous, are lower level than Heal and less useful in general).
I'd recommend that Blessing of Leeches grant some extra or improved benefit, just not a very powerful improvement/addition. Perhaps immunity to poison as long as the spell's active (i.e. the leeches suck the poison out of their blood too quickly for it to take effect on them), though that's very limited in usefulness. Perhaps increase the Fast Healing to 6 or 7 HP instead. Perhaps grant immunity to pain, stunning, and staggered status while Blessing of Leeches is in effect (i.e. the leeches' painkiller effect is boosted by the spell, making the character numb to pain, almost unaware of it, so unhindered by pain).
