80 % of the time a used cool head dome is not correct for the build. Its the wrong cc has been cut too much etc. The other 10 % of time the shell is worn. And the dome sits flat to the shell.  Unless its a smoking deal or you know its good and the rite dome. Your going to spend 40 bucks for a new dome. And your so close to new price its not worth it.
Biggest benefit of them is versatility. Like ken said if you waste a piston. Its only the dome  not the shell too.
 If you have a good clean head to work with now its not a huge benefit.