The big drawback to Mega evolutions is that you are only allowed one per battle. Imposter ditto's drawback was that he would never become anything that you could not get otherwise (ie if you have him transform into a Salamence, you could have just included a Salamence on your team and not have to content with the HP defect). But if you have one Mega-Evolution, and your opponent has one mega-evolution, then you can use an imposter ditto to effectively have access to two mega evolutions.
This is assuming of course that ditto will transform into the mega version of the opposing poke. --- I understand my username has an error. Just live with it. However, to see my real signature simply hit alt F4. http://myanimelist.net/mangalist/Xero127