I am beginning to understand that now. But what you said is just an excuse for a lousy combat system. Or more correctly it's implementation.
I understand that it's their game and thus the result of their decisions and design. But there is also something called "generally accepted standards". In this case it's a standard of quality. They knew there are going to be differences between server and client side and yet they implemented this system without specifying in tool tips or anywhere else that those skills are supposed to only be used in groups as they are almost ineffective in solo fights. Either they didn't expect the system to have such discrepancies or they did and done it any way. In either case it's bad quality that is the result.
Every other MMO out there has to work with server/client communication and yet they manage to offer you a combat system where "what you see is what you get". Why can't it be in this game as well?