I have fixed stocks on a couple of my more "general use" ARs, but I have a collapsible stock on the one I use for home defense. I set it to a very short length to work with a squared-up close quarters shooting position, which also makes it more maneuverable in the house... my HD plan involves me covering the approach to the stairway to the bedroom areas of the house by leaning around a corner in the hallway, and this is more easily done with a shorter weapon. The same goes on a shotgun... for a fighting shotgun, I like a real short stock, for all the same reasons.
Collapsibles are fine for close quarters positions with red dot sights, but I do not like them on scoped rifles. When fully extended, for use with traditional field positions, your cheek rests on the bare buffer tube, which leaves your head that much lower, and gives you that much crappier of a cheek weld. Also most of the popular collapsible stocks (the factory M4 stock included) do not adjust as long as an A2 stock, even when fully extended... which is the length I need for use with field positions.
I realize that a UBR or F93 stock would help with these deficiencies, but it comes at the cost of much greater weight. Any rifle I own that has a scope is a rifle that I carry and use in the field, and weight is a concern. I just don't have that pressing of a need to be able to adjust the stock on field use rifles. With a fixed stock, I have a NTCH position with light clothing. If Im wearing heavy winter clothes, my head simply rests a little further back on the stock than usual, just like on any other rifle, and it's really no big deal.
Having said all that, I do not currently own plates. If I get some, I may revisit the usefulness of something like a UBR stock on my 2A-purpose rifles that currently wear fixed stocks.
