I always laugh when someone rebuys at my table. Here's the thing.
Maybe you are on the early stages of the freeroll. For some reason (maybe a bad play, most often bad beat), you got kicked out. You can try for a rebuy, but is it really profitable here? I honestly can't say that it is. At this point you can't be sure at all of whether you'll finish in the money. The tourney just started, you're as better off as if you had just sit in. So, in this spot, it's just too big a risk.
If you are on the medium/final phase, it's even worse. I see people re-buying for the original 2000 chips when the BB is already at 400. They are forced to play a shove/fold strategy right from the get-go. Even if there is a little gap (say, BB is at 100), they are forced to play tight and enter fit-or-fold mode. And most likely, if they get lucky, they will reach the lowest prizes. They will have to survive for a little more to even recover what they spent, let alone profit.
From my point of view, it's a waste. You enter free of charge, if you get kicked out, then go. Freerolls are not that much about skill. They are about luck (early stages) and aggression (late stages, and even then luck plays a major role). You can't change any of these things by re-buying. I tend to do pretty well on freerolls when I start out playing tight and then get to 200-500BBs or more. I can confidently sit out, have dinner, go for a walk, come back, and still have at least 100BBs to play and push people around, after all the all-in freaks are gone. If you don't get to that spot on those early stages, then you're done for the tourney. That's the way freerolls go. Just go out and try another time.