Reel spools

New fixed spool reels are generally supplied with a spare spool, either a plastic or metal. The plastic ones tend to be of a lower capacity which I believe to be for short range match fishing. Is there any other reason, other than possibly price that a spare spool would be plastic. I am thinking along the lines (sorry) of winding a spare spool with braid, and don't know whether there is any difference in the type of spool to be used.

Red
Most of the reels nowadays seem to have plastic spools. Often you will find that the second spool is smaller but there is no real reason that I know of apart from what you suggest.
IMHO you generally buy the reel to suit the fishing you'll be doing, so if you're casting long distances you'll need a larger reel with larger spool capacity.
As long as you have sufficient line on the spool for the distance you are fishing at plus some more for playing big fish that run then you should be ok whatever type of fishing you do.
I don't know how experienced you are but you must be careful not to overfill the spool or you will find yourself in a terrible mess as loops of line fall off the spool causing chaos.

in general line or braid will be taken from a metal liped/spool faster,aiding casting and improving distance depending on the anglers ability,so for that little extra always use metal spool and yes companys supplyan extra plastic spool due to cost.