I don't know that I'd agree that designing to use existing parts is "idiotic". In fact, I'd say it's genius.
That said, there are likely design considerations that one should take into account that you may not have to when designing from scratch.
IIRC the KAST "midlength" in their 5.56 guns is simply the 7.62 gas tube re-purposed. However, if you're going to re-purpose that gas tube, you can't use the same size gas port as everyone else using a traditional carbine or mid-length tube. You gotta spend a little more engineering time to make that standardized part work properly.
Maybe what you mean is that Glock's method of doing so can be idiotic?