Mega Salamence is pretty much one man army because he can heal, cure status, Dragon Dance, and kill everything with 30% Flying-Return STAB all without a need of help from any other. Put that on top of his already high stat at defend, attack and speed and you got yourself a unstopable train of kick ass.

The fact that he die doesn't matter to the rest of the team mean the other Pokemon can be focus on cleaning after he wreck the shit out of half of enemy team.