I think of Stifle as a reasonable Late-game card actually. Blocking the SFM trigger, the Snap-trigger, a Jace Ult, or even a Liliana-sacrifice is pretty nice. It "feels bad" because your opp got a 2-for-1, but they got the shit half of the deal. They didn't get the spell they needed from Snap and they didn't get an equip, but instead just a Squire. The more I play it, the more I find strange outs.
The other day I won a life-race in part by Stifling a DRS activation. Seems bad to essentially say "I gain 2 life.." but that's the thing, the card's versatility: hitting Terminus, hitting Fetchlands, hitting a Grim-activation, hitting the BSK trigger, stopping a Pyro trigger. Lots of times it costs you a card for "nothing"; but you were about to lose a bunch of cards or lose the game. The fact that it acts as DRS 5-8 some games, and acts as "randomly hose your opponent's most important game plan" other games has felt quite good.
I know that I sometimes would land an SFM and they'd spend a stifle and my lizard brain would say "Got the 2-for-1!!" while I was being beat up by a Delver; but it matters. I get that Therapy can hit all those equips too, and more reliably a lot of the time, but it can't stop the Batterskull if you get the Therapy on the wrong draw step, it can't stop the Snapcaster from killing your last threat, or the Terminus from sweeping your board.
As always, no problem if you just disagree, but I really feel like in the past month or two I've really come around to the card as a versatile game-saver. It feels a lot better than I would've thought to stifle the Snap trigger and leave your opponent without the removal they were planning for, or without the Ponder they needed to find card X.

