My argument is that a spammer is more likely to spend that extra CPU time versus a legitimate visitor - if the option were there. I'm not gong to sit around for a few seconds waiting for my comment to post. Comments are pretty much bullshit anyway (on most sites), so I generally won't post them and contribute to the garbage pile. But If I did, I certainly wouldn't be around waiting as if I was on some 20 year old dial-up connection.

The stopping of spam has more to do with the spamming scripts not having javascript interpreters and not being able to do any calculations at all rather than the artificial time penalty. The most beneficial implementation of PoW is with a lightning fast calculation that won't even be noticed by the human visitor.

And this too can break accessibility for some people. Not all browsers have javascript (a text-only browser like Lynx comes to mind). I'm not a fan of using Javascript for something that isn't likely to benefit the viewer and isn't optional. I do like the server-side evaluation of the content however.