We require 5 rune keepers because costs must remain low for beginning players so they can obtain runes easily, and also because there has to be ample opportunity to receive "junk" runes. Without that, players progress too fast and things stagnate.

I never said I actually like the idea of paying gems for runes, just that if the feature is to be there at all it's too expensive as it is currently implemented, given what you receive for your (current) 200 gems.