Why is because its a value judgment, just like when you say that that person has a "great figure", etc. (which by the way hints at a personal bias).
I agree that more elaboration may be nice, but personal reviews are inherently subjective and the fact that someone found a girl to be "too skinny" may provide useful info to hobbyists. It is after all, simply one opinion of many, and presumably this lady has pics in the public domain. I also happen to think that members are intelligent enough to take one liner posts like that with a grain of salt.
But mostly, I have a big problem with stifling honest opinions. Creating an environment that encourages only rainbows and unicorns is incredibly short-sighted. It fosters short term gain for SPs but ends up costing everyone in the long run through lost integrity and trust. You don't have to be a genius or a long term hobbyist to have seen that in the Toronto scene in recent years.
If you want to get into a discussion about posts of dubious value, pm me or let's meet for a beer. We have much bigger problems than asshats who post "mean" one liners.