gaskets vs sealants?

Pardon me if this is a newbie question...

I recently replaced a valve cover seal since it was leaking. Of course, 
now it leaks as much as ever, even with proper torquing and a new 
gasket.

Are people strictly using silcone type gasket sealers nowadays, or is 
there some disadvantage to those?

Thanks!
Steve
71 TR6
72 GT6