A rebuy has its place. An add-on is a "protection racket"...(ie it's dodgy as ****).
Let's start with Rebuy. I don't mind letting people blow themselves out, restock and blow up again. To me, that puts money in the pot. Great for me. For the re-buyer, it gives them a second chance. So its win-win and increases the total number of paid positions (or higher paid positions)..
Add-on's though are a con that we should all be pushing back against. The big problem here is that it devalues the position of the tourney's winners so far. There is often a large change in the purchase value also, with the add-on being cheaper than the original buy-in. All of a sudden, those with a solid strategy find themselves having to pay again just to stand still. That is unjust.
poker sites should not be constantly looking for more ways to make a higher percentage from each game. (I think they are charging too much as it is). Antes and rakes are too high too. (Go on, change my mind!)
It is interesting to me that, when I was a bigger noob than I am now, I loved add-on's. The reason is that in one fell swoop , I was back in the game, with a middling stack without having earned it. Now that I play much more and generally am faring better around the add-on period, it is more of a hindrance to my success than a benefit. Essentially, whatever edge I had is erased. If I choose NOT to buy the add-on, then I am moving backwards. So that means I am FORCED to use the add-on facility, or accept the handicap.
So no. Don't like the options that are solely there to increase table revenues without adding to the game.