depends.. on the occasion if its a wedding there is often a cost to the host, it is courteous to respond. I had couple people not show up at my wedding who said they would, it would have saved me a few hundred bucks each if they told me ahead of time.

if its just a bbq or a party, and youre not feeding people, then its not that important. I usually let them surprise me.

With email invites, I find people sometimes only half register them
Ive had people forget it and ask, oh yeah when is that?
(even though they can easily look it up) ...

in general though, if there are people who dont respond, and dont show up, I dont bother inviting them in the future.
p