I agree completely with Mark's point - the 3880 is a much "tamer" beast than other large-format printers, especially used ones (it's darned easy to wreck a big printer moving it). A 3880 is a 45 pound machine (you can actually pick it up and move it, and it's not very big with the feeders closed) with a very good sheet feed mechanism - most of us use sheet paper much of the time, and it's hard to feed sheets into a 24 inch printer. All current 24 inch printers are 100+lbs, stand mounted machines.
I also agree that the sheet/roll cost differential is very dependent on the specific paper (and generally less than it was a few years ago). There are even a few papers that are cheaper in sheets - I don't use GFS, but some Epson paper is cheaper in sheets (especially with rebates), and the Moab line is about the same price, while Hahnemuhle tends to be quite a bit cheaper in rolls.
Unless you're making mostly large prints (the width of the roll), sheet paper will require much less trimming - the 17x25 issue is one of the few cases where roll paper is actually easier. Cards are a particular pain on roll-fed printers (I've owned several), while I can give the 3880 a stack of 20 Entrada cards and walk away from it (I'm not yet brave enough to leave the room altogether, but I can certainly do something else).
The only thing I don't understand is with all the 3880s out there (it's easily the most popular printer on most fine-art printing discussion boards, and Epson reps have told me that it outsells all the larger printers COMBINED), why more paper manufacturers don't make 17x25" paper especially for the 3880 (and 3800). They know it's popular, it's the first printer they profile - why not make the odd-size paper it needs?