For us, it comes down to this, we are DVC members, our DVC is paid in full, our annual dues are still less than what we would spend to stay off property, especially in a similar hotel/condo. Let's look at our trip for next month, cost in the most comparable room I can find (remember to include parking, a kitchen, internet, pool, jacuzzi, all non smoking rooms, etc.) would be $2,966 for our dates. Compared to annual dues for the year at $1,122.13 and I will get two to three vacations off that.
Can I get a cheaper room? Sure, I absolutely can, but it won't come close to the size of the DVC room. I can find something of a similar size, but doesn't have laundry in the room, etc. for $851 a night plus they charge $10+tax per night for parking. If I were paying out of pocket, I would consider it as an option, but you would need a rental car on top of that.
Add in the Disney touches on property, the refillable mugs, etc. and it just works well for us, but everyone is different and again if we didn't have DVC, I would have to seriously reconsider staying onsite, especially after Magical Express ends.