Tapes are relatively cheap, and it's not worth the risk of losing your data. Use multiple tapes in your backup set. If you need to backup your data, you can't afford to trust it all to one tape. If you lose that tape, or it gets damaged, your backups are worthless. I'll probably get some flack for this, but this is my policy.
Incremental backups can work very quickly and make things easier during the backup process.
The problem comes into play when you actually need to retrieve data off the tape. You'll need to have your tape software re-create catalogs by reading through all your tapes. If you have incremental backups on a tape, this process can be very time consuming, and sorting through the results very confusing. The last thing you want to do when you're rebuilding a crashed machine is to have to pour through your manual trying to figure out how to restore your data. It may take hours or even days to get a consultant out to help you figure it out, and that means hours or days of downtime.