Firstly, I do not support the tactic for the most part, as it would only make logical sense in a rare few circumstances, as my example later in this post shows. However, I do recognize it as a perfectly legal tactic by the rules.
Storyteller01, creatures summoned through Summon Monster and Summon Nature's Ally spells do not truly die. When they would die, they disappear and return to whence they came, but they do not actually die. Unlike Calling spells, Summon spells either create a temporary duplicate of the summoned creature's body (not summoning the creature itself, but a copy of its body and essence), or they otherwise fiddle with reality in such a way as to summon a creature that doesn't actually die when it dies (perhaps summoning the creature from an alternate potential timeline, or whatever). It's in the PH as far as I know, under Conjuration (Summoning) in the Magic chapter.
A Summoned creature doesn't truly die, it just returns to whence it came, and lives on. No demigod or whatnot is likely to care about this, or else I should think that if gods did care about Summoned creatures (who don't truly die), they would have already killed all mages with knowledge of Summon spells and destroyed all records of such magic. Regardless of alignment, any mage can cast Summon spells if they learn them. No deific power is going to care if a good guy Summons temporary creatures to be killed by that good guy's ally, no more so than that deific power is going to care if the bad guy Summons a temporary creature to go die for them by attacking the good guys. Whether the Summoned critter is a celestial, a fiend, an elemental, a pixie, or whatever, it doesn't matter, nobody cares, the Summoned creatures are only partially real or at least only real in some unfulfilled alternate-reality timeline that never would've come to pass anyway.
Besides, why would some demigod or whatnot see the bad guys as enemies, and be angry at the PCs using creatures they Summon from the demigod's domain for the sake of being the party Fighter's AoO-Cleave bait? The Summoned creatures would be just as dead if Summoned to attack the bad guys directly, and would be far less effective if used that way. Why wouldn't the demigod or whatever take offense at the bad guy Summoning monsters from that realm to attack the PCs or something? Exactly, it makes no sense, so nobody anywhere in the D&D multiverses cares about Summoned creatures; they're not real enough to be cared about, and they aren't really truly dying. No Cleric of Pelor (or any other compassionate deity) would dare summon anything for the purposes of fighting evil if he summoned creatures were real beings, actually suffered, and died for real.
Summoned creatures go nowhere when they die, except right back to where they were before summoning, because they didn't really die and they just go on living in whatever timeline/dimension/nonreality/whatever they existed in beforehand. Dead PCs will not be assaulted on the Fugue Plane or whatever by angry ex-Summonings.
Storyteller01, you missed something said earlier. The primary use for this tactic with a fighter and a caster is when fighting tough enemies who have high Spell Resistance and/or saving throws and/or energy resistances, making them more or less immune to the caster's spells. Summon Monster and Summon Nature's Ally can be stopped by Spell Resistance. The first time a summoned beastie tries attacking a foe, the summoner must roll a caster level check to beat that foe's Spell Resistance (if any), and a failed CL check results in the summoned beastie disappearing without any harm to the foe with Spell Resistance. If the summoner succeeds on their CL check, then that particular summoned beastie doesn't have to worry about Spell Resistance against that foe anymore (except for purposes of opposing the summoned beastie's own spell-like abilities or whatnot, if any).
Using the tactic discussed in this thread, the party's fighter can dish out more damage with the caster's help by AoO-ing very weak summoned beasties (all it takes is a Summon Monster I, or maybe a Summon Monster III to bring out a handful of such frail beasties if the fighter has Combat Reflexes) and then using his free Cleave attacks against the Big Bad Evil Guy. If the PC fighter has reach, then the PC caster can just summon his creatures about 15 feet behind the fighter, with a command to attack the fighter, and then the fighter will get AoO's against the critters as they pass through the fighter's outermost reach, with a weapon like the ranseur, longspear, or glaive. Critters from Summon Monster I will only have a few hit points, and the fighter's Strength alone will probably make his minimum damage sufficient to drop the summoned critters. If the party is fighting an enemy that has little or no Spell Resistance, then of course the PC caster is better off summoning a powerful critter with Summon Monster VII or something, which can then dish out decent damage and take some hits that might have otherwise been directed at the fighter. However, big summons don't help when fighting an enemy who has high Spell Resistance, just as blaster spells don't work well against high-SR enemies.
As for the whole ally/enemy thing, the PC caster can always choose to consider the PC fighter as an enemy for purposes of his summoning, and direct his summoned creatures to move in such a way as to provoke AoOs from the reach-fighter. By the "Rules as Written", the cleave-mooks-to-hit-BigBad or AoO-mooks-to-cleave-BigBad tactics are useable and workable. From a logical standpoint, no, they certainly don't make any sense. The Big Bad or whatever is doing absolutely nothing at the moment to let down his guard and draw in a sword through his gut, regardless of how idiotic and helpless his henchmen are being. It also doesn't make sense that the PC fighter should get cleave-attacks against the Big Bad just because he AoO'd the PC caster's summoned weenies, for the same reasons.
But you have to realize that AoOs by their nature aren't terribly logical as to how they work in 3.x D&D, so nothing that deals with them is going to be entirely logical either; no reason why you should get extra attacks in a round just because someone's prancing around you like a moron as opposed to you just charging in and swinging like a madman to kill everything in your path. So you have to try and not think too much about attacks of opportunity during play, or else you'll just hurt your brain trying to wrap it around the inconsistencies and illogic of it all.
Now, the only way I can partially rationalize this nonsense is to consider that those weak little mooks are being clumsy and getting in the Big Bad's way when they do whatever it is that draws AoO's from the PC fighter. The mook lets down his guard by charging the ranseur-wielding PC fighter, then the fighter takes advantage of the reckless movement, spearing the mook in the arm as he starts to move into and then through the fighter's outermost reach.....then the fighter just continues his momentum and steps forward a foot or two, driving the ranseur's tines into the Big Bad's shoulder too as the mook either dies (if it's a relatively-frail mook) or blusters past with the gash in his arm and completes his charge against the fighter. The Big Bad couldn't see the ranseur coming at his own shoulder because his mook was in the way, and he didn't expect the fighter to lunge like that when his mook charged forward so recklessly. It's harder to rationalize with critters the PCs summon, though.