That's not true, compare Draw 7s in Vintage to Draw 7s in this deck and then look at how much more acceleration this deck has, it should be clear that the card is functional. Whether or not it's less functional than in TES is up for debate, sure TES has Chrome Mox and SSG compared to 4 more land, but this deck also has 4 Tendrils of Agony and 1 Empty the Warrens compared to 1 Tendrils of Agony and 1 Empty the Warrens and Mystical Tutor + Street Wraith is a "full" tutor if you draw into it and Ponder is far from dead on the combo turn. It's also not like you have to win the turn you cast Diminishing Returns, it just has to put you in a better position to win.

I just don't know how you guys are actually approaching this deck, I mean Mystical Tutor -> Diminishing Returns, Lion's Eye Diamond, cycle Street Wraith and then sacrifice Lion's Eye Diamond in response is one of your biggest plays.