I believe it can put extra strain on the belt.
If it's in good condition then it probably won't make a big difference.
In my case we weren't sure how long ago the belt had been done or if it had been at all so we didn't want to tempt fate.
Replacing a belt is one thing, replacing the engine cos the belt snapped is another.
It turned out when the belt was taken off that it didn't appear to look that old from the outside.
The white writing on the outside was still fairly bright and readable so it looked like it hadn't been put on too long ago which would indicate it had been changed. However on the inside of the belt the rubber was all cracked.
The mechanic reckoned it had been changed at some point but the belt that was put on was old stock that had been sat on a shelf for a long time before being fitted so it was just as bad as sticking an old shagged one on.