I had a tantrum shinedown which I rode for about a year. It was 29 front 27.5 rear 160mm front ~150mm rear. I now have a ripmo. I bought the tantrum frame because it was inexpensive and it was before stuff like the ripmo came out and I didn't like the switchblade.
Anyway, I thing a mixer bike is fine but I think it's always going to be a niche for bike geeks. The simple fact that you can't swap tires or inserts or anything front to back is surprisingly annoying. Maybe on my home trails I run something like an aggressor front and a ikon rear, but I want to go to the bike park so I want to move the aggressor to the back and put a DHF on the front. Oh wait, I can't, I have to buy 2 new tires. Same issue if you are going on a trip and want to bring a spare tire. Now you have to bring two spares. It's not a huge deal, but it's a bit annoying.
For the OP's scenario, it's a cheap way to "upgrade" to a good part of the benefit of a 29er, but it does have the downsides I mentioned before.
Also regarding the nimbleness and the rollover stuff... Honestly I think the nimbleness mostly comes from the front tire. The rear roll over is probably more noticeable than any nimbleness you get from the back. Think about it: you want a smaller, lighter wheel to make changing direction easier. In the front that wheel is pretty much attached to just about nothing and you can feel that with your hands and arms pretty directly. Take the weight out of the rear and...well the rest of the frame and shock and all that stuff are most of the mass anyway and it's just less noticeable. It matters less.
Yeah the bike can get slightly shorter, and that's probably the biggest thing to make it feel more nimble, but if you are 6' tall like me, that probably isn't a big deal. Maybe it is for a shorter person, because the front end is shorter so as a ratio the shorter rear may matter more. If you want better roll over for stuff like roots and things, why do you want that only at the front of the bike? Why not have it at both ends?