There is ZERO reason to waste bandwidth complaining about the quality of a web comic. I can (and do) ignore most of them without anyone's help. PVP, on the other hand, is awesome, and everyone should know about it.
Back when content always cost money, the purpose of a critic was to tell you what was good and what was bad so you didn't waste your hard earned scratch on crap. That's still more or less true when it comes to things that you can't try out for free. The thing is, there's not much you can't try out for free these days.
With the free model in place, what does a critic have to do? Well, in the age of the internet, a role of critic gives way to that of an advocate. There's no longer any point to criticizing as everyone has access to the same (or at least adequate) content that a critic does, normally for free. The new problems is that of sorting the wheat from the chaff, and that's where the critic/advocate again becomes useful. There's hardly any use for 'bad' reviews these days. I can find things I hate on my own, thanks. What I need is a concentrated dose of what's awesome. What that is amazing would I possibly miss if the advocate doesn't alert me to it?
People seem more naturally predisposed to rant about what they hate, but that's of no use in the internet age. Useful information centers around what's great and why. Provide that, and you're contributing.