As somebody wanting to jump on full frame...
In terms of slow events, fashion shoots, and even general videography, an APS-C sensor is mooooooooore than enough for everyone.
I only carry two lenses with me these days, a 28-90mm Vivitar Series 1 and a 50mm Nikon Series E, and since my D3200 is APS-C, those lenses are effectively 42-135mm and 75mm respectively. Yep, they're very very odd focal lengths but I've never had any issues with it out on shoots or whatever.
Honestly, the only reason why I want to switch to full frame is simply because of a wider field of view. I'd be able to get closer to subjects and get some decent depth of field which is what I really want.