I heard they explode inside the camera if you re-use them x amount of times.
Seriously though, you can supposedly use them many times if they're high enough quality. The higher the quality the more sturdy the media, meaning the more resistant failure from stretching/wear caused by pinching and shuttling. I'm sure climate/moisture has it's effect as well.
I agree with fevercity... if it's worth shooting it's worth a new tape. On the otherhand, if the shoot is #$%#$ I have no problem reusing an MQ63 or even a PQ for dog tricks. The media can handle it, nobody gets hurt. No big deal.
Oh yeah... and they're a great way to save media. Very cheap, timecoded, last a long time if stored properly. Another reason not to reuse them.
NOTE to self... LABEL LABEL LABEL. I can't tell you how frustrating it is looking for a "reel" 3 years after the fact amongst 20 others without labels. Time in the trash. Time which may not seem valuable today, but in 3 years may be worth a lot. Give future you a break and label everything well, including your log notes in your editor.
I will never re-use a tape for any reason. A few years ago, we shot a short with a Canon XL-1. We used a used tape for one of our name actors scene in a beautiful old time gunshop. We had drop outs on the tape. If it is worth shooting, it is worth a new tape.