I'd like to challenge that. Even if keys were unique for each build they are stored somewhere and a third party installer could just as easily extract them. You would need to obfuscate keys by hand at random positions for each release for this scheme to slow down a third party installer noticeably.
It amounts to a pretty standard copy protection scheme, and all of those are broken not very long after release. (By people who receive by pay by the way, defeating this installer would be worth money.)