The limitations on team types for various events is part of what makes the game more interesting than other games of this type. This is, largely, a collection game. Team restrictions are what encourage and reward breadth of roster. Without those restrictions, you'd just be able to go all in on one team and steamroll everything, and it'd be quite dull, with little incentive for collecting. Yes, there's a money factor, but it's also the fundamental tool for shaping the game experience.
More unusual teams, like mixed-faction or mixed dark/light teams do, as a consequence, have more limited application, but there are still quite a few places to use them. Cantina, arena, galactic war, territory war, mythic events, raids. There are places to put it to use. But you'll still need specific faction teams and multiple functional teams for a variety of reasons.