"Talk-driven development" I've read a few posts on this, and i've decided i'm firmly against it and will downvote any conference speaker who does this (and any conference which allows it). It's a disservice to conference attendees and other speaker candidates who didn't make it.
Yeah. Sometimes it works out and the deadline of having a conference to present something new serves as powerful motivation. Other times, I have embarrassedly rolled out an old talk, and that (as you say) isn't fair on those who missed out.
I need all the discouragement I can get.
(I'm serious, actually. I do it far too much, and it's stressful for me too.)