This falls on both sides, imo. With it being so easy to send out hundreds of "e-vites", as opposed to the effort/expense involved in sending nicely printed or handwritten invitations by mail, I can't be sure that the invitation was actually intended specifically for me, with any modicum of thought involved, as opposed to just being an email blasted out to someone's entire address book. So I tend to only respond to the ones where either I actually am going, or I have some reason to believe the sender wanted me in particular to come or give regrets if I can't (e.g., if it's a close friend, or I know I'm part of a smallish group of invitees).

I do agree that not RSVPing and still showing up is a breach of etiquette; a non-response ought to be treated as an implicit no, imo. But to help that along, it does pay to make sure to explicitly request an RSVP fairly prominently. Among the flurry of emails I get, an invitation that requires an RSVP, versus an open invitation that I can just show up to or not, aren't always that easy to distinguish (for example, I don't have to RSVP to show up to visiting speakers that my university sends me email "invites" to).