>>50566623In a nutshell, Mega Banette will be strictly inferior to Mega Gengar for lower DPS/TDO, but still have beastly damage output.
Mega Banette will be worth maintaining in your roster as a max level mega, for whenever Mega Gengar isn't available. Think Halloween events where you'll probably want ghost megas running every day.
Banette's situation does illustrate a problem though, many of the mega evolution Pokémon yet to be introduced are so painfully weak that you wouldn't consider using them in battle; they would only be worthwhile for their field bonuses.
These include things like Audino, Medicham, Mawile, Aggron, and already include Steelix, Kangaskhan, Slowbro, Altaria.
Mega Aggron is completely inferior to regular Metagross.