I don't think any Megas are "pointless", yes, its obvious that there are Pokemon used mainly for their abilities (Nidoking, Azumarill, Serperior, Megas, etc.) or a period of time, Mega Alakazam was becoming more prominent, and regular life orb Zam was popular. The meta is always changing, and you don't really know what Pokemon will start being used more for whatever reason (to counter or check certain threats, for gimmicks like Gothitelle, or just simply because people want to experiment) For instance, Amoonguss and Tangrowth are unfortunately in OU because the metagame is leaning more towards stall :/
And for Aerodactyl, its usually better to have a Mega Aerodactyl than just a CB Aero. Yeah, Choice items can boost stats more than a Mega evolution, but if Choice Band Aero would pack a bigger punch most of the time, then it would be at least in the same tier and usage as Mega Aerodactyl in UU. Regular Aero is RU lol. Choiced items are good, but their disadvantage is you can only use one move.
Pretty much every Mega form of a Pokemon is better and used more than its regular form. I'm not saying that regular forms can't work, but usually Megas are better... and since Megas are so standard now, you're technically at an disadvantage without a mega. But its not impossible. For some Pokemon, it may be better to hold a regular item instead of a mega stone for certain situations, teams, and of course to mix it up. Certain pokemon like Alakazam, Life Orb or Sash can be useful, and so can its Mega form. Trace could come in handy. Again, it just depends on the scenario.
No mega is truly "pointless" in my eyes; sure there may be gimmicks and only a few reasons why they're used, but thats how it goes in Pokemon. Its like Smash; you make whatever works to work.