Any reason to keep castform in my dex? Future evolutions/upgrades that would make them relavent?

It's pretty much just a fun little Pokedex filler with a neat weather gimmick. If you think you could trade some of the more elusive forms to someone who won't see them ever (e.g. snow form for someone who lives closer to the equator) then they could be worth keeping. Otherwise you won't really gain or lose anything by ditching them, save some box space.