No Save is inherently superior to higher saving DC on this scale.
Please, Durkon's touch AC is probably below 10. A cleric of at least 17th level with even a 10 on Dex is far more likely to hit him than Durkon failing his saving throw. Besides, moving parts are irrelevant once Blasphemy immobilizes Durkon, because he is no longer... yanno... moving.2) It doesn't require any attack rolls and it's not restricted to melee range. Fewer moving parts than Harm or Disintegrate.
Marginally valid, but not really a speaking point against it in the face of the possible consequences of failure. It's extra resources spend, but unless you plan on going on another dungeon dive that particular day, not one that is notable. Remember, you gain spells every day during prayer, but dead foes are dead... well, not forever, but at least until they can get a True Resurrection.3) As a (probable) domain spell, it's a less flexible option to burn; he wouldn't have been able to convert it to a spontaneous infliction spell (though spontaneous infliction is probably less valuable than spontaneous healing).
But he already has it in reserve. In fact, if anything, this gives him more options, not less. Let's say they were hiding out. First off, they'd better be more than 40' out or (except Belkar), they just landed in the same boat. No save. At that range (outside Sneak Attack range, so Haley's damage is negligible), the only viable threat is V. Who has a notoriously bad Fort save, being an Elf (with a Con penalty) and a Wizard (with a poor Fort progression). So now is when you cast Implosion on the Wizard, finish off the mooks at your leisure on successive turns, then return to dealing with the still paralyzed Cleric you were about to gloat over at your leisure.4) If Durkon has any hidden allies ready to spring into action once hostilities commence, Redcloak's got a powerful spell to squash them with.
Uhh... Blasphemy leaves him unable to do anything for minutes. No saving throw permitted. You can do whatever you like with him in the meantime, and Disintegrate the corpse. Guaranteed kill. As opposed to the coin-flip saving throw of Implosion which leaves a now potentially hostile Cleric free to either disengage or counterattack. We aren't talking Implosion vs Disintegrate or Destruction. We're talking Implosion vs Blasphemy, then follow up with something else that leaves no trace. It's burning several spell slots for a guaranteed kill vs burning one higher level spell slot for a risky kill that leaves your enemy able to immediately respond.The only real argument against using Implosion is that the target doesn't suffer any adverse consequences if they make their save - though the damage from a successful save against Disintegrate (5d6) or Destruction (10d6) is going to be unremarkable for an adventurer of Durkon's level, anyway.