From experience I can say that you can re-use DVCAM tapes (the 124 minute ones) for at least 100 times before they will start showing dropouts (which is what we have to do at work)
However, I was told that re-using tapes a lot will wear out video heads faster so it's stupid to do so (and I believe the guy who said it, he's a genius in everything considering broadcast).

I don't think that re-using DVCAM tapes 3 or 4 times will do much harm and I can assure you the tapes can take much more before they show drop outs.

I've tested the reusing of MiniDV Panasonic 83 PQ tapes and found that around the 10 to 15th pass the tapes starts to show considerable dropouts.
I reuse tapes 4 times before I consider them garbage.

Dropouts are a mysterious thing.

I have had my pd170 for a long time and it had no dropouts for the fisrt 2 years. My vx2100 is brand new and has audio drops about one every tape.
The pd170 has dropouts when when the cam starts rolling anywhere in the tape. The vx2100 has drops happen anywhere at anytime regardless of when the cam was started.