1. No, unprofessional is doing a crappy job. Providing crappy service is not the same as doing a crappy job. (e.g., I'll return your call in a day vs. right away) Yes, it may still be unprofessional. But, if they don't want to treat me like a professional, why should they expect top notch service?
2. Unless you're talking about high-volume incalls, girls don't just sit around waiting for guys. They take appointments, and run errands and what not, in between calls.
3. Even if the girls only want to work 5-6 hours, how much of a discount would you need to ensure that she is booked back-to-back for that period?
4. Some girls are also selective in who they see. In order for that to work, scheduling has to accommodate the SP and the client.