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 :(

EDIT: This one's not it, but it's very similar: http://motivatedgrammar.wordpress.com/2011/11/07/descriptivi...

I speak British English and it's more a matter of register for me. If I'm writing something formal I'll use the subjunctive, if I'm talking to friends I'll just stick with the indicative.

If I saw someone write "it is necessary that he goes" in a paper it would stick out as strange to me, likewise if I heard someone say "it is necessary that he go" it would sound overly formal and a bit stilted.

So really, you can use either and be understood perfectly well. It only seems to annoy prescriptivists, and I would say one has a moral obligation to annoy prescriptivists anyway.

"was" sounds correct to me (US), but everyone learns a slightly different language model.

Use of the subjunctive mood is slowly declining, especially in the UK. We often smooth out the difference by inserting a "should" or "might". E.g. "It's important that you (should) be on time."

Peeves make TERRIBLE pets. Set them free; you'll be glad you did.