Another plus to bus transportation to/from SSR is that the bus stop at most of the parks is very close to the park entrance. If you drive, you may have to take the tram to the back end of the parking lot. By that time, you could very well have been back at SSR, depending on how busy the bus stop was. Not to mention that it's very easy to get turned around driving in WDW after dark. The signs aren't really easy to see, and you are manuevering around all the people heading off site.
We used SSR buses during F&W festival (over a weekend), and only filled up a bus once. And that was at EPCOT after Illuminations, and as they filled that bus up, another one pulled up to take the extra folks, so we didn't wait at all.
I can probably count on one hand all the times I've driven to a park this year when we stayed onsite. And I've spent about 20 nights there this year.