From the point of view of ACT, the free licence policy does benefit supported customers in indirect but important ways. By making the software itself available to all, it leads to a larger user pool, and therefore more and better trained programmers, unrestricted opportunities to test the software before adoption, and an enthusiastic user community whose work contributes to the overall quality for the product. But it does not obviate the need for direct support from ACT.
Because customers are responsible for delivering a reliable and efficient software application in a timely fashion, they cannot live with unpredictable third party black boxes in their system. In addition their job is not to be an expert in the core technology they are using but rather rely on someone else's expertise for that, while they can concentrate on their software mission which ranges from flying a plane, to controlling a set-top box. GNAT, being free software, allows them to build a black-box free system. This greatly reduces risks and improves time-to-market since the customer has permanent, end-to-end control of all the components present in their final application and has direct access, via a support contract, to the the developers of the software they are using.
GNAT is an open technology, which means that anyone could provide support services. The fact that sources are widely available means that there are many people who know the technology well. Many of them are working at ACT, but there are many other very competent people who know GNAT well, some of which work for or with customers. This is one of the great advantages of an open technology. By contrast with other proprietary technologies, where there is no possibility of any expertise developing outside the vendor. Sources can be put into escrow, but very often the sources are not in sufficiently good shape to be usable to anyone.
The GNAT sources are among the best documented and maintained code in the industry. ACT knows that its code is going to be carefully examined by a large community of Ada experts, and therefore they have to be sure that they like what they see, and find it easy to work with GNAT. Several University research groups use GNAT for projects involving modifying and understanding the GNAT code.