Power Ups like Assists?

[The Future Warrior]

Thing is, when it comes to assists, having multiple lower tiers of them active at once gives more targets in a battle, which forces your opponent(s) to dive their attention and strength dealing with them. That said, if you don't want to have multiple weaker assists in use...don't? Buy a stronger one. Higher levels allow you to have multiple weaker assists out, or a single stronger one of the appropriate tier.

In regards to power ups, yeah being able to use more than one at a time would be useful and give variance, but it would also create a lot of paperwork and bookkeeping, and be much more expensive. And in the end, you're still only one target, so objectively it's not any better or worse than just using a single, strong power up.

You're welcome to do a flavor thing and say that your higher tier transformations are just using multiple lower-tier transformations simultaneously, but mechanically it would still be one, higher-tier power up. S'just the way the system works.