The screen on a phone is simply too small, even with a keyboard. The stripped down versions aren't just stripped down because of the slower processors; your fingers are too big to be able to select all the features. Instead the portable apps let you make minor fixes in the field. I have fixed a typo on a powerpoint via my phone, but I would never want to try to do any, say, spreadsheeting on a phone, even with a BT keyboard.
A tablet, well, I still doubt it but I can imagine being wrong: perhaps it's a tablet game changer. We'll see.
The reason I think Microsoft won't make that mistake again is that they already tried it, both ways (full-ish apps on windows phones, and a phone interface, metro, on the PC). Both bombed, and though I'm not sure Nadella really does know what he's doing, I do know he knows that repeating that would be a mistake.