I've never bought a rebuy or an add-on.
If you bust out, and rebuy, suddenly you are back in the tournament, at (presumably) well below average stack size.
This makes no sense to me, so I just leave.
Add-ons might be worth it, but I don't know how to do the math.
I'm guessing ICM has something to say about this.
'twould be a good topic for a poker strategy article.