A good explanation from Scott (hippies), slightly edited:
Excess head wear is caused by 90M (DAT) tapes due to the deck not being able to properly tension the tape. The differences in tape tension can cause friction on the heads. Think about a chain and sprocket which is not properly tensioned, or a belt and pulley which is loose and slipping. If you look at the problem this way, it's easy to see how the added wear / friction can occur.
The transport/ drive motor is also effected because of the added weight of the tape spools carrying 1/3 more tape (mass) than the deck was designed for. The fact that pulling 90M tape will force the motor to do 1/3 more work each time a 90M tape is used shortens the motot's lifespan. Hours are not so much the factor here as the excess load during actual working time.
But Scott adds:
All that being said, I have run 90M tape for years. I don't want cuts in my music, and am willing to wear the deck out faster as a result. It's a tradeoff as i see it.