One way to go around this is when you join a firer team and all the slots are taking up by AI. It will kick the AI and replace it with you. So that way if you want to play with who ever you can and not have to worry about AI taking up the slots.

That's what I had in mind and then I mentioned in my other comment that there will be people who prefer AIs and that they will get annoyed and leave when their AI buddies get kicked which will in turn annoy everyone who just wants someone to play with and then people will complain. Destiny is supposed to be a social game. Friendly AIs tend to be unreliable in video games or in other cases they are overpowered. If they were overpowered, then no one would play together. Why should you play with someone who is bad at smash bros when you can just have a level 9 cpu on your team? People, that's why.

Good point, there's no real way to balance it. And if they did people would abuse it. Never thought of it like that.

I think the option to have teammates that you can control still outweighs the negatives. Maybe allow only AI teammates or human teammates?

It would be a nuiscance. Even if there was a way to allow one or the other or if a player joins he takes the place of an AI, it will annoy some people. Destiny is designed to be a social game. Having computer controller fire team members is detrimental to the whole thing.