It's big among co-sleepers and also among the minimalist set. I don't exactly know what's anti-minimalist about having a bed frame (lol) but I can certainly see the benefit for co-sleeping -- if babe rolls off, it's just a short roll rather than a big fall. I guess the minimalist perspective could be that if you have no room to store anything under the bed, you avoid the temptation of storing anything under the bed?
We are fairly minimalist but we have a bed frame (not an actual headboard or foot-board though). Where I grew up, having a mattress on the floor was considered really... gosh, I don't want to say "ghetto" to offend, but considered basically something really poor people had. Now, I don't look at it like that for other people, but for me I still have the residual mindset having grown up in a really poor neighborhood and seeing that a lot -- so we keep the bed-frame.
Also, some people have a problem with mildew and such under the bed, and also vacuuming/sweeping tends to be more difficult because you can never get under the bed unless you lift it, which is a pain -- then random dust can accumulate at the very edges.
Other than those considerations, I think it really has to do with personal preference.
ETA: as for comfort level, for me I don't think it would be very comfortable getting in and out of it being pregnant and post-partum because I'm around 31 weeks and it's already getting to be a bit of a hassle getting up and down off the floor (playing with dd and so on). As far as sleeping, I'm sure it's probably fairly comfy.