I know what it does, I use it to categorize stuff I email to myself (+reading, +reference, etc.). I just don't see how it would help filtering against some who also knows what it does. Maybe they don't show up in the list because they stripped it out already.

It's a feature of gmail to provide unique addresses. It might protect your other accounts that use your base e-mail address from an automated attack vs. the compromised data. It doesn't matter much if the database goes public, though.

You can still use the "add as many dots as you want" feature in gmail, even if they don't allow the plus sign.

so.me.l.ongus.e.rname@gmail.com

Someone should make a gmail plugin for decoding directives this way.

It doesn't cost you anything, and few things piss me off when I'm signing up for a service than when it won't let me use an RFC-valid email address because it has stupid validation in place.

Why wouldn't they?

I do this with almost every website as a way to segregate or block mail if needed. It's pretty eye opening to see what companies trade or sell your email addresses.

example+ebay@gmail.com and example@gmail.com point to the same GMail inbox; you can use the part after + sign to filter incoming mail. A quite useful feature, I must say.