For my family onsite is always worth the extra money. I am not so sure it is more expensive for every resort onsite though, because by the time we factored in a rental car, parking at the parks, fuel, we are already ahead. If we add convenience and onsite benefits, there is no comparison.
I think that every family has their own preferences, but in some cases onsite is less expensive then off. A friend of my DD wanted to plan a WDW trip, and gave me her dates and travel party dynamics. I found a FD discount at a moderate, and costed the trip out, including money set aside for tips. On the previous trip they had stayed offsite, and had not dined in WDW restaurants due to the price. My total price was less than her previous trip and allowed for meals and tips, and no driving or moving luggage.