I love using the WDW bus system, but it seems to be my observation that you may be able to drive yourself in, even from off property, and get in the park just as quick, or even quicker than taking the bus.
When taking the bus, you may get lucky now and then, and walk up to your bus stop to find the bus that you need, waiting and ready to go. You walk right up, load up, the door shuts and off you go. However, that is probably more the exception than the rule. It has happened for us before, but here is also what happens to us sometimes as we use the bus; we get to the bus stop and watch bus after bus come and go. Say we're going to Epcot. Here comes a bus for Magic Kingdom. Next a bus for AK. Then a bus for DTD. Next, maybe another bus for MK. Then a bus for DHS. Finally, an Epcot bus. We have stood there and waited probably 15-25 minutes, watching bus after bus come by before ours finally comes. Then, once you get on the bus, depending on where you are staying, you may have to stop at the rest of the bus stops for that resort, possibly stop at another resort, and then finally get to the park. Of course, there are also those times when you have to wait for the ECV to load, which can take 5 minutes or so. Not criticising those in ECVs, mind you. If you're handicapped your handicapped, and I am thankful for my health and that I don't have to be in one of those. But they do add alot of time to your wait.
Then, on the way out of the park, depending on when you leave (especially if you are leaving at or near closing time), you may get in such a long line for your bus that not only does it take forever for your bus to come, but when it does come, it loads up and fills up and drives away while you are still standing there. You may even watch a second bus come and go, before you finally get to load up on the 3rd bus. That whole process may take half hour/45 minutes as well.
I'm not knocking the WDW bus system, but those are simple facts to consider. If you stay offsite, you walk right to your vehicle and go without having to wait for the ECV to load, or for the next bus to come after the first one fills up, etc. You can literally drive in and park and be into the park in about the same time, or quicker, than it would take the bus to get you there after you factor in all the things I just described.