My thoughts- im all for it as there are many user names that will go either way. when i say hello to a guy its more like " whats up dude/Man/Bro...." when i say hello to a lady its more like " good morning "Name", whats up "name" and so on. the big reason is when refering to someone and calling them a He when they are a She or backwards.

now when picking gender as some have stated above ( Jenn, Kat) you should have a choice. there are men that believe they are the wrong sex and women that believe they are the wrong sex. When some one has the XXY gene they tend to lean twords one sex and would like to be referred to that so if you are able to choose Male or Female and thats how they will be addressed.