Posts that ive seen with hidden comments have them listed at the very bottom, making them pretty easy to miss for most people on long posts.  

I guess what i have to wonder is why should we allow someone to control the replies _that other people see_ to their posts.

Like my post about the  flags, and the reasons i thought you were  wrong -- wouldnt it have been kind of shitty if i could have just completely hidden your rebuttals (not that id do something like that, but as an example).  And yeah, people could click on the greyed out thingy to see what you said.  But some people. maybe even most people wont.  The whole purpose of the greying out/putting it at the bottom, is to make it less visible.

Just as an example, a couple weeks ago, i noticed a top-of-trending post that was completely plagarized.  I wrote a comment, a couple whales noticed and the post ended up losing most of its reward.

Would it have if the poster had the power to hide critical comments?

But even beyond downvoting, the ability to hide dissenting comments takes away the incentive to rebut them.  It detracts from engagement and debate.