If you want to be convinced to stay onsite, then ask this question here. The people who frequent this forum tend to be great fans of staying onsite. If you want to hear a more varied response, then ask this question on the Orlando Hotels forum. There, you'll find a lot of people like me, who have done both, onsite and offsite, plenty of times each.
I would say that there are certain circumstances under which you would PREFER staying onsite. And others underwhich you would PREFER to stay offsite. Here are some who might strongly prefer onsite:
--- notice and like visual illusion/details of a theme throughout a resort
--- don't want to rent a car on vacation and won't have their own car with them
--- don't like to drive while on vacation, preferring mass transit - bus, boat, monorail
--- their group may split up, perhaps with teens or babies or elderly folks
--- only plan to visit Disney
--- very early risers on vacation (early entry) or enjoy the late night hrs at MK (extra fee)
--- shop a lot in the parks and want package delivery to hotel gift shop, not just to park entrance area
--- want to charge purchases to hotel room key, not to regular credit card
--- want their checked bags sent to their hotel room, not picked up in airport baggage claim
--- want a themed pool slide (moderate & deluxe only)
--- don't mind added cost of onsite hotels
--- want costumed staff and reminders of "Disney" around them all the time
Everything else that one could want or enjoy onsite is now available offsite at a lower cost, especially in the last 5 years or so.
My DH and teenagers all think that "Disney" everywhere gets tiresome and it's a hyped grab for the wallet. We drive our car even when staying onsite and we've found it faster to drive to a nearby offsite resort than to wait for Disney transport and walk from bus/boat stops to our rooms. We like to sleep in a bit and we love the luxury amenities and restful atmosphere of a top timeshare resort condo with privacy for all of us, extra tv's & bathrooms, washer/dryer, jacuzzi and a kitchen for our hungry teenagers. Beautiful landscaping, poolside musicians or piped music and fun activities under the Florida sun round out our Orlando vacations. We enjoy a variety of theme parks and beach visits as a family together (rarely split up), not just Disney's. So we don't really fit the description of the Disney Resort lover. There are many more like us, but you probably won't find them on this forum.
If you plan to visit WDW for a relatively short visit and you don't mind all sharing a small hotel room for that time and/or you only plan to visit Disney parks and/or you won't use/have a car, then you would probably enjoy an onsite stay a lot - especially a deluxe or moderate hotel. The value hotels don't have a hot tub and they feel rather like a busy, active, colorful Holiday Inn to me. Little kids love them! I would prefer a really nice condo vacation (at the same price) anytime. YMMV.
Btw, mousekeeping is just housekeeping. Disney changes the names of some things to make it sounds more "Disney." We've gotten both very good and very poor housekeeping service quality - both onsite and offsite... even towel animals offsite. Same goes for the quality of guest services, other hotel staff, restaurant staff and park personnel. There's absolultely nothing to the idea
(anymore - years ago there was) that these things are
any different at Disney, IMHO. These days, many other places in the region pay better and even Disney has trouble retaining quality labor personnel - oh, excuse me, cast members.