I use a back quiver always have. I haven't been field shooting so I can't tell you what if any advantages over a side quiver. Personally I like the feel of it, its out of the way so when at events and walking around you're not going to have your arrows sticking out and hitting people, I've seen people with side quivers positioned that whenever they walk the quiver spins and the arrowscan hit anyone close.
If on a crowded shooting line it can be easier getting your arrows out without hitting people, and they do have an advantage when speed shooting.
It'll take a bit of practice but after a while you should be able to draw your arrow holding the knock and put it on the string without looking in one smooth movement. You can;t really do that with side quivers as you have to pull out the arrow away from you and depending on where you've grabbed the arrow you might need to canter your bow to put the arrow on.
Like others have said you can't bend over too much otherwise arrows may fall out, but teaches you to bend at the knees, which is better for you're back anyway. Also yes you might drop some missing your quiver, but again bit of practice and you can put them in without even looking.
From the
longbow shoots I've been on, back quivers are not as popular as other designs, probably 3-4 out of 60 archers, though they often have very nice leather work done on them.