Quoted By:
Look at it like this:
When Mega Blaziken was revealed, people were calling that it'd be broken overpowered.
The reality is, it's just an alternative way to use Blaziken, some people still prefer to use Choice Band or Life Orb because it's simply better or their play-style is focused around it.
People seem to forget that in order to get a Pokemon to Mega Evolve, you need to sacrifice their item slot for it; and for some Pokemon, they become infinitely more useless without certain items unless it gets replaced with a worthwhile ability.
In my opinion, a good Mega Evolution makes an otherwise weak Pokemon more relevant, or it adds an alternative way to play an already useful Pokemon.
