A GCD is not the only solution to preventing using multiple skills at once, period.

Further, I thought it was great that STS made PL as an MMO with an arcade feel, multiple skill use or not, -- it's why I played.

I don't understand why some think it's a valid argument to say "other games do this blargy blarg." If innovation were bound by rules of what already existed and what other people did there would be no innovation.