Deena said alot of the good reasons, already for staying off-site. I particularly like staying near Downtown Disney when we do off-site. Having access to a near-by mall the other way doesn't hurt, either. There are plenty of eating places in that mall that we enjoy and you have a supermarket there even though it's high priced. If you go the other way towards Kissimee then getting on Hwy 192 is a no brainer to get to Disney, as well. We are not in our rooms very much except to sleep. I'm there to do the parks, not the resorts. By the time we are done with our day I probably could sleep in the parking lot; so, it's not a plus for me to stay at the very best of the best considering what you have to pay for the honor.
When we bring our grandchildren I love getting a condo or renting a house. Everyone can have their own space. I know you can do the same at Disney but we do it for a 1/3 or 1/2 the cost depending on the time of year. I rather spend the money saved for a nicer meal or souvenirs or doing something extra like Disney miniature golf, Wonder Werks, etc. By saving on the higher resort costs at Disney it has allowed us to extend our visit by a few days which is the best perk of all. Obviously, you have to have the time to do it.
There is a different slant on staying at Disney property since
9-11-02. It's alot more on security with the cars as you get into each resort for checking. This is especially so at the value resorts. I'm not sure if this has lightened up but last October this probably caused an extra 1/2 hour each day; so, staying off site might have been a real time saver at that time. Supposedly, there were threats to Disney World; so, there were extra precautions taken. We always do the car thing even on-site. I don't want to be on a crowded bus or monorail, etc. I don't find that to be fun at the end of a day.
With all that said I would make a few exceptions to the rule. If you get All-Stars for $49 a night I might still stay on Disney. We did it for that rate but we still had EE. The other time it would be better to stay at Disney is if there was alot of construction going on all over off property. A couple of years back Hwy 192 was a mess. It probably tripled the time to commute. The answer there though would be go in a different direction and you could still stay off-site. Fortunately, I've never seen it bad in every direction at the same time.
If it were my honeymoon or 25th anniversary which is coming up for me then I would opt for a Disney Deluxe resort regardless of early entry. Those events put a different slant to it all. Another time I would do it would be for Christmastime but I may stay at a moderate or value resort. Also, if it were my first time I think staying on site might be acceptable but again at the value resorts. Now, if money is no object then by all means stay at the best of Disney resorts. I'm not fortunate enough to take any vacation without looking at the bottom line. I pride myself on getting the best value possible for what we like and for what we get out of it.
This debate of staying on or off has been going on for a very long time even when EE was still available. The character bus will never be a reason for me to stay at a Disney resort but you should know this is what has taken the place of EE.
Finally, I've done so well with economizing it has allowed us to take multiple trips per year to Disney. I rather go twice and stay so so then to go once and stay at the best. Everyone has choices. I'm not a resort person; I'm a park person. It's that simple.