I agree with rwmastel. I can't remember where I read it, but regular cleaning of tape drives is damaging to the drives themselves. Companies that regulary clean their tape drives have more tape drive failures than companies who only clean the drives when the clean light comes on.
I think it has something to do with the way the cleaning occurs. If something is brushing on the readers (I'm not very technically experienced here) over and over, it will eventually wear them down. Cleaning only when there is a buildup helps lower wear and tear.