Specifically, how "clean" the low end tracks during fast palm muted parts is the main issue
When the power section starts sagging/breaking up, the low end "smears" and everything sounds loose - it can get bad enough to where the drummer can't keep track of where you are rhythmically, especially at high tempos
This isn't a big deal if you're playing classic rock or doom, or other genres - in fact it's DESIRABLE in them
but if you're playing metal, hardcore, punk, where the drummer is consistently hitting 105-110 db, you're going to need that headroom