Descriptive linguistics doesn't mean "whatever anyone says is correct and it should go." This ain't 'nam, there are rules, and you can still run afoul of them. The basic tenet of descriptivism is that, as people who study linguistic patterns as a science, we can't make a value judgment about a certain set of rules or what a person "should use". However, I think it's generally accepted that we try to strive for some common set of agreed upon linguistic rules when trying to formally communicate on the Internet, and many would claim that the subjunctive mood would fall into that.
I saw a really good write-up about this before and wish I could find it :(
http://en.wikipedia.org/wiki/Descriptive_linguistics