I think it could be an incredible mechanic/feature, personally.
It will definitely NOT just be somehting as simple as just pushing a button and your boy Noob Saibot throws a clone as you're fighting. Neatherrealm always tries to innovate.
What I'm thinking is that the Kameo fighters, while maybe not fully fleshed out, will actually be more playable than some people are thinking. What I'm saying is, I think when you "tag" them in or whatever, they will fight alongside you and have their own moveset that you control. For those that played Tekken Tag 2 - it could be something like that.
I'm super excited for this. There are so many possibilities. Also, for the 5 people on the planet who want shitty characters like Kai and Dairou to return, this could be a way for them to be in the game in some form. Having said that, if the Kombat pack is 6 full characters and only 5 Kameos, it's possible they are way more fleshed out and intricate than we may think.
I cannot wait to see the gameplay for this.