In recent years it seems there has been a big backlash against this term by feminists claiming the idea itself is mysoginist... Personally, while I am sure the term has been used by people in sexist or downright hateful rants, it is a perfectly fine term for situations that could apply to people of any gender, but, as you have observed, tends to apply more to guys being "friendzoned" by girls.
I don't have statistics to back this up, and it may be influenced by media portrayals as much as real life (although as we all know, art imitates life and life imitates art), but from all my experienceds by impression is this:
1. In male-female friendships (assuming they are both attracted to the opposite gender), it's more likely that the guy will be interested in the girl romantically or sexually than the other way round. By this I mean that, in the absence of other obstacles (e.g. both are single), it is more likely that the male would be willing to start a relationship (or even just sleep with) the female than the other way around. So there are a lot more cases of guys that are friends with girls they would be interested in doing more with than the other way round.
2. Girls in general seem to have a more binary distinction between "romantic interest" and "just a friend" than guys do. In addition, this is not just a case of having friends that you would never be interested in - it's the "I don't think of them that way" factor at play, not just the fact that the girl would never be interested in them anyway.
For those reasons (which, again, are just my suspicions about reality based on anecdotal evidence rather than anything I have statistics for) I think the "friend zone" is a useful term, and though it can cut both ways it more likely applies to the standard guy being in the "friend zone". Also I would consider it basically a descriptive term that can apply to a bunch of different scenarios, rather than one that assigns blame to one person or the other. I am sure scenarios could exist ranging from both extremes, from the blameless angel unknowingly friends with a vile mysoginist who is basically a rapist and believes being "nice" entitles him to sex, to the evil bitch manipulating and stringing along the heroic, sensitive guy and deliberately taking advantage of him and what he does for her while banging the biggest douchebags in the universe just to make him feel bad.
Saying someone was in the friend zone (or whatever else you call it) doesn't have to suggest the relationship is one way for the other. Basically it just seems to be used whenever someone is friends with someone but interested in them romantically while the other person isn't, though personally I think its only really meaningful when part of the reason for the lack of attraction from one of the people is because of the friendship, ("If you don't ask her out soon you'll be stuck in the zone forever", to paraphrase the episode of Friends that helped introduce the term to popular culture).
Probably bigger post than the OP warranted, but I have seen discussion about "friend zones" turn into shitstorms elsewhere so I thought I may as well just get my opinion on that argument in...