Why would a communicator need to do anything other than serve as a phone? Updating the Tri-corder as a smartphone makes more sense and I guess you can say they should combine those two things but I know from using a phone it takes a moment to get to the phone part of your phone. You don't wanting to be messing with your communcator trying to get to the right app when you got a Mugato trying to kill you. You just want to press a button and talk into it and have someone beam you up.
Jason
You're kidding, right?
Ok, let's say you lose contact with Lt. Jones who has the tricorder, and you need to know about your surroundings. Now what do you do with your communicator?
Again, TOS was made in the 60s. They didn't foresee all this. Just think about what you can do with an iPhone, and all the separate devices it replaced. And you're asking what someone would need 200 years from now?