Aneros

Yeah it can touch the mattress and limit movement. Removing the tail also makes it easier to use sitting in a chair. I've done it with a lot of mine.
But the tail is useful for keeping the device pointing straight, so the p-tab is less likely to move off its spot. That's why I leave it intact on a lot of my models.

Yeah that's pretty much it, if you're arching your back and "getting your butt under you" or just laying down with legs straight, then the back tab can touch the mattress and hinder movement. 

Personally I enjoy the feelings that both tabs can provide as my Aneros is pulled totally inside! The flexible tabs of the Syn models provide lots of stimulation! 

I like both tabs on the trident design models, and the Progasm. I've never found them to cause any problems for me, personally, but that was a different story with the older models with the curly tail handles. I have an original MGX and Helix, and I had to cut the tails off both. Those tails would press against the bed if your session found you on your back, and you definitely didn't want to relax yourself to sleep with that tail pressing. 

Just a personal note, I was making progress towards the Super-O, but I wasn't there yet. I cut the tail off and polished the MGX, and BOOM! it was one factor among many, but I had a totally mind melting, head explosion, life shaking orgasm right after the modification. It felt like it hit a lot deeper, and maybe traveled more.

My personal experience with the K-tab is that in addition to stabilization, it limits the total pressure on the prostate once the Aneros is pulled inside hard just before the orgasm, and helps with sort of softening the pressure during chained orgasms. I found this when comparing the Aneros Helix Classic and the Trident version. The classic can make the prostate a bit sore after hour+ sessions. Of course, it depends on what you are going for.