I've been riding for donks, started off road riding in 70's and never used to wear gloves until I started doing big days/milages, - mainly to get some extra padding between bars & palm. Used to have some nice original 70's string back fingerless mitts....er hmmm.
I started wearing gloves nearly all the time after a mate came off on a high speed road descent and shredded his palms - not a pretty sight....and trying to pick bits of tarmac, car window glass & dirt out before bandaging him up...not good!
After a high speed off myself whilst riding my mtn bike through a section of loose rocks, I now wear decent long finger gloves. I was wearing fingerless gloves at the time, and one of my fingernails got quite mashed (at the time, the least of my worries!!), but I could see where the palm on one of the gloves had taken a lot of abrasion, so I think it prevented injuries to one of my palms - shame about the rest of my body!
At the end of the day (and having worked in a hospital A&E Dept), I think with any 'adventure sport' it's up to participants to take on some responsibility and precautions for themselves - and for mtn biking, wearing a helmet, gloves and glasses seems sensible to me. Oh, and it might even save you a lovely evening sitting in an A&E waiting room for hours (Don't get me started on binge drinkers wasting A&E Depts time!)