Probably not, but it depends on what you are looking for.
We don't like to rent a car, and shuttles from off site are a bigger pain, so we "miss" Disney transportation to our hotel room when we stay off site. For us, its the deciding factor for the on-site/off-site debate.
Another factor is travel time. Generally speaking, even the Downtown Disney hotels are a little harder to get to than the on-site hotels. If you are planning on a mid-day swim, there is nothing like being a quick monorail ride or walk away from your room - but you aren't getting that access with the Moderates anyway.
Disney hotel rooms do provide special programming on their TVs to help you get the most from your vacation. It isn't specatular, but its there. Also Extra Magic hour is available to on site guests (and maybe DTD guests as well?). It used to be that Disney resort guests got preference on Priority Seatings - they could book further out, but this hasn't been the case in years. (I keep expecting this to come back as a cheap perk to give people to encourage onsite booking).
You can get a lot of the resort magic by booking some meals at the resorts. i.e. - want to check out the animals at AKL, book at meal at Boma. Always wanted to see the monorail go through the Contemporary, book your character breakfast at Chef Mickeys. The theming is what really sets Disney resorts apart from their competition, and you can taste the theming by doing some resort visiting. Of course, that takes time that you could be spending in the parks.
With the possible exception of AKL rooms (where you can see animals from your Savannah view room) Disney hotel rooms are hotel rooms. There are bigger and nicer rooms offsite for the same money, but the offsite rooms don't have Mickey on the soap. Some will say that the level of service from Disney Cast Members (the hotel staff) is better on average as well - and that is probably true, on average. But there are great hotel staff members off site, and some Disney staff that could use an extra sprinkle of pixie dust.
It used to be, back before the internet, that staying onsite gave a little extra security in knowing that your accomodations would be nice. Now, you can check here and make sure that the hotel you've chosen isn't a fleabag motel.
