If the Cleric uses Astral Seal in most turns, the party lacks one damage dealer. In a large party with several strikers, that's not much of an issue, but in a small 3 PC party that would prolong the fight by several turns. And a longer fight means more time for the monsters to do damage, so it evens out.
If you consider the effect over several encounters between extended rests, it means that the party will have more healing surges left over. Unless your party runs out of healing surges often, that doesn't make a difference. It makes no difference whether you go from 4 healing surges to full or from 1 healing surge to full.
Another issue with Astral Seal, and all other temp-hp granting at-wills for that matter, is scaling. PCs start with around 30 hp and go up to at least 150 or even 300 at 30th level. To scale properly, Astral Seal would have to grant at least 5 times as many hp at 30th level. Let's see: A level 1 Cleric can grant 2+Wis+Cha, so with Wis 18 and Cha 16 that's 9 hp. At 30th level, the same Cleric would have to grant at least 45 hp to keep up with scaling. Even with a 26 Cha and a 24 Wis, he's only at 17 hp. He would need to add some 30 hp healing to make up for the gap.
The effect of scaling is that at 1st level, Astral Seal is very good, but its usefulness gets less with every level as the amount of healing you hand out is less and less compared to a PC's total hp and the damage dealt by monsters. And even with a dedicated healer build that effect is hard to counter.