Availability Strategies in MS SQL Server 2008
Database Mirroring isn't a database recovery strategy.  It's a high availability strategy.  If you can afford to loose up to 15 minutes worth of data then you take transaction log backups every 15 minutes.

If you need to look into high availability that's a totally different conversation, but it doesn't remove the need to be taking transaction log backups.

As for your self-hosted customers, they are responsible for setting up their own backup solution unless your contract states that you'll work with them to set one up for them.