I asked myself: would I rather have a group of all martials, or a group of all casters? And except for some edge cases, in most battles I would rather have all martials. If you get extremely lucky on dice rolls a group of casters could win a tough fight, but it's far more likely that a couple monsters make their saving throws, they attack the casters who are trying to concentrate, and the whole thing turns into a rout. A group of martials is going to take a lot of damage, but they are also going to pump out a lot of damage, and overall have a better chance of winning. (Once again, my opinion.)
But of course what I really want is a mix of the two. Which kind of suggests the game is working as intended.