Travel agencies are not legally allowed to hold money for clients in a "layaway" type plan. Anytime a client calls and says I want to pay $XX towards my trip, that payment is made via a debit or credit card (or gift card or Disney rewards card) immediately to Disney, which is not any different than if you did it yourself except that you didn't have to spend the time online or on the phone dealing with it.

That money is no more tied up than if you paid for 1/2 your trip at booking. Either way, to cancel and get a refund you'll still have to wait for Disney to credit your account or gift cards, minus any cancellation penalties that may be imposed by Disney.
For my own trips, I always pay the deposit only and set aside funds in a savings account until final payment, but plenty of clients want to make "payments" along the way and for some people, that works better for them.
-Astrid