Joseph-Nowell are very strong defenders in their positions, but as a 12-13 combo, both would be out of position. Personally, avoiding seeing that is the biggest reason I can see for having a 12 on the bench.
As for Croft -
He's been back playing for the last two months; this isn't a rush-back scenario like 2013. If we rule out players for not stringing together 6 games more or prior to international return, we will struggle to get a team out at times. People are letting his nightmare 2 years colour their perceptions here.
He carries well. He's not an ice breaker carrier like Vunipola, but he regularly makes lots of ground when slightly wider, like Morgan. He rucks well. Go look up the Ruck Mark series on Demented Mole (and while you're there, have a look at Corbisiero's numbers for next time people assume he only scrums). Notice the very easy comparison with Lydiate. He works hard - he made 1/6th of his side's tackles against Ulster, give or take - if you want him to work closer to the coalface than Johnson did, he's actually rather effective. As for having our best lineout jumper on the bench along with Tom Youngs' "Will he, won't he" act being a luxury, well, that's a really interesting definition.
Hopefully he doesn't have one of those games where the coach tells him to stick by the tramlines and we might be one step closer to sanity...