Most hotels that do participate in park-bound transportation contract with a shared service, i.e. one bus will stop at a number of hotels. Also, for the most part, there is no alteration in the schedule - if there are three buses in the morning and two in the afternoon/evening, but both return buses are full, they do NOT add additional buses. The guest is stuck finding their own transportation.
The Marriott properties at Little Lake Bryan provide transportation, but I don't know if it's one of these shared-routing things.
The six/seven Hotel Plaza Boulevard properties do provide regularly-scheduled, all-park transportation all day and evening during the hours the parks are open - every hour or half hour depending on the park and the time of day.
Generally, though, it's advisable for guests not staying onsite to have a rental car.