None of that stacks up with freeing both the mage rebellion and Redcliffe from the Venatori. It helps that I don't really care what happens to the templars all that much.
Saving the mages is a noble goal, but it is not worth undertaking a suicide-mission, given the Breach (unless it is your only option, which it is not). If you fail in your mission and die, many will suffer (including, but not limited to, mages), and the Breach will destroy any ulterior political goals you hoped to accomplish by supporting them anyway. If you care about mages, you should care about the general populace and their security too, because you are really not making anything better by helping mages if as many or more people are suffering to accomplish that.
And does Dorian's ability to reverse the spell really wash one's hands of the failure? Something to think about...
As for the brewing threat of Alexius and the Venatori ... well as the saying goes, "Another day." Or "Can't win 'em all." When you see where you're outmatched, the smart thing to do is walk away and try to find some way to even the odds for next time. In a sense, recruiting the Templars does that. You gain a powerful force than happens to specialize in neutralizing hostile/unstable magic. Now you've got an answer to the Venatori when you have to face them next. Walking into the lion's den is just not smart strategy. Like, ever.
It's not like liberating Redcliffe ends the Venatori threat, anyway, you just deal with less of them later. I personally find Red Templars a more formidable an opponent to fight (facing Knight Captain Denam at Behemoth status is hell-on-wheels) and would rather nip that threat in the bud than Venatori, but that's just personal preference.