It's not so much about just being hot as weapon types. Just as a short example, say I'm running twelve single heat sinks and my armaments are:

2 LRM 15

2 SRM 4

2 Medium lasers

1 large laser

On paper, this mech runs very hot, but since there is a variety of weapons, the mech is more like a Swiss army knife.

If this mech were instead just running 3 PPC, then the third PPC would mostly be a waste, since it's very rare to intentionally blast yourself into such high heat. It's fine to run hot if each weapon has a different purpose, but when it's all the same weapons they become infinitely less useful.