Because RBAs can be recoiled/rewicked in one sitting, then you can put them in a baggie or a little box or whatever (to keep them from getting debris inside them) and carry them around with you just like you would carry a pack of drop-ins.
Also, like everything else, RBAs don't last forever - things break, screws/parts get lost, etc. "Too many" RBAs means you have parts to cannibalize when you lose/break parts on them. Sure you can probably buy screws that fit/work, but you won't find them at a big box store. So in order to buy the right ones, you have to know how to measure them accurately as well as figure out what the thread spacing/coarseness is - not easy to do, especially on such small screws.
If you're willing and able to build/rebuild, RBAs are golden in a vapocalypse situation. The more, the better, IMHO.