Why would I ever merge my plants?

This is a genuine question. If I have to spend weeks to unlock one merge, and it ends up being level 1 while my current game progression has me fighting higher-leveled zombies, why would I ever merge my plants? It's the main mechanic of this game, but it's actively discouraged because of how weak merged plants start out when unlocked.

The easy solution would be to base the level of the merged plants on their components. I don't know exactly how it should be calculated, but any attempt at it would be an improvement over the current system. There also is the option of removing plant and zombie leveling entirely, which puts all unlocked merges on a fair playing field.