what's the point of bringing incapacitated add ons/perks?

I'll answer your question separately for each thing that inflicts Incapacitated:

The killer perk Eruption: This lets you know if people are working on generators when they explode from downing someone (after you kick the gen). If they were, they can't continue working on it for 16 seconds. This is not a bad slowdown, but Eruption itself is decent because at the very least, you are delaying them a bit, and in the best case scenario, you can then follow up with perks such as Pop Goes the Weasel, or block the generator with Thrilling Tremors as you have forced the survivor off. It acts as an information perk as well as a slowdown. Even if you don't bring any other perks to complement it, you have an idea of where to go to find somebody after you hook the survivor you downed.

The Huntress add-on Weighted Head: Yeah, you're absolutely right about this one; this addon is not that great. More often than not, if you hit someone with a hatchet, they are not going to be completing objectives anyway unless you sniped them from really far or something like that.

So, to answer your question, yes, Weighted Head is indeed probably not worth it. Eruption is alright.

I wouldn't mind this change. As of right now it would only affect weighted head anyway. Having Victor or being chained by Pinhead (the other things that inflict incapacitated) already prevent you from leaving anyway.