In R+A tournaments you should always purchase the add-on, especially if its large compared to a starting stack. Your first goal is to make it to that point, and while its not a goal in itself to rebuy, you should do it if nessesary. The only times, I dont, is if I want to end my session early, reduce my table count, or I am on tilt.
As some have said, it can be a reasonable strategy to register in the last minute, so you only have to play a few hands before the add-on. Also note that some R+A tournaments only charge rake of the initial buyin. If you are an MTT player, this is an offer, which is simply to good to pass on. So if the choise is between for instance a 2,2$ R+A and a 5,5$ regular MTT, then always select the 2,2 R+A first.
The biggest caveat of R+A tournaments are, that they require a larger
bankroll. Since you are always spending the add-on and often one or more rebuys as well, the 100 x bankroll rule dont apply to R+As. You need to view a 2,2$ R+A more or less like a 5,5$ regular MTT and have at least the same bankroll.