It's not mandatory.... And there is strategy in joining.

I think GDI is fairly balanced. It cost you money to be a member. It restricts the amount of hits you "want" to make on a single country. It restricts late set hitting.
It still allows pest like ops like tech stealing, food/oil burning and bank bombing....

Maybe once the few of you anti GDI protesters run non-GDI country and have a good chance making a top 10 finish and get blasted by some random nobody just to drop you out of it you will have a much different opinion on the matter.