Well, Zone said for someone to suggest this, so suggest I go.
The reasoning behind commenting on positive scored reviews is simple: it may have exaggerated/biased/flaming comments behind the positive score.
For example:
"X server is so good because it has biased GM's, overpowered donation items, GM's banning you for no reason and all that fun stuff! So if you're an idiot, you can join this server! (rate 90)".
Obviously an exaggerated example, and it would probably be reported via the report button very quickly. But people are unfortunately sneakier than that, and may drop subtle but false bits of info within positive reviews.
Just my 2 cents~