1. Mountpoints are good if you are running out of drive letters so to speak. This is more likely on a cluster because SQL Server instances cannot share the same disk(s) in a cluster - in a non-clustered environment all your SQL Server instances can use the same disk(s).
2. You would still keep your OS on the local drive, wouldn't you? Otherwise you will have to boot from the SAN.
3. If disk space and performance is an issue on the local D drive; move the backups to the SAN as well (assuming the performance is better). However, because there is also a Disaster Recovery aspect, you might want to keep them on the local drive; your data is on the SAN and you don't want to put all your eggs in the same basket (if the SAN fails, your backups are still available if it is on the local drive).
MCITP | MCT