There is absolutely an advantage in that they can "wait" for DAS attractions without waiting in the standby queue. This is by necessity, but it frees them up to watch a streetmosphere show or even a major entertainment offering, shop, eat, and generally take in more of the park, while the non-disabled guest is still physically in line.
The only alternative to that is the "holding area" idea which wouldn't work for a variety of reasons.
While there is the advantage, I don't consider it unfair because 1.) it is required by law to accommodate guests with disabilities, and Disney is known for going above and beyond to be inclusive of these guests, and 2.) guests with disabilities and their families/travel party face challenges which the average, non-disabled guest does not face, and in many cases, can "slow them down," which can impact the amount they can experience (in both quality and quantity) during their visit.