Well if your just going to parks from your hotel yes it's pretty straight forward, but the bus system is so strained that wait times can be pretty bad and many bus routes make quite a few stops. In this case I would much rather take a very short bus ride to the closest park and then take a quick monorail ride to whatever park I wanted to go to. The biggest issue with the bus system is hotel to hotel, and yes a lot of people do travel between hotels. Dining at hotel restaurants is very popular just try to make a reservation and you will see this.
This is a little outdated, but gives you an idea. The monorail system capacity is around 200,000 per day the current average bus ridership is around 160,000. The current monorail system is easily capable of handling the full average ridership of buses.
Disney currently has 72 monorail cars, I would say that if they expanded the system to somewhere around 100 to 150 cars they could easily increase the capacity to somewhere around 300,000 to 400,000.