I think there are plenty of reasons not to use more personal information.
1) First and foremost, people may just not want to or prefer to remain private.
2) Secondly, employers or potential employers are checking forums, social networking sites, etc., more and more frequently. So, if you are using more personal information I think there is a certain amount of responsibility that goes along with that, or it may force your to carry yourself more professionally. That is unless you really don't care. Personally, I don't really have anything I would care about hiding. I don't carry myself much differently than what I would in the office.
3) In the case of potential employers, they might do a search on here and find and everything you said during your interview is a sham because you're on a forum website trying to either study up for the interview, or your history shows you have only an adequate knowledge of SQA. This could cost you a job.
4) Maybe you don't want your online persona to be taken as being your real life persona. Some people are more over-the-top online and aren't that way in real life.
I'm sure there are other reasons, but I think it just boils down to personal choice, and I think a lot of it is simply creating an online persona that you want without being compared with your real life one.