While AA is not technically a mod, it does use the UT engine. [/b][/quote]
Very true, however so does several other games as well (http://wiki.beyondunreal.com/wiki/Un...ngine_Versions) - each of them being their own, individual, game.

There are only few good 3d engines on the market that most others utilize and 3d engine is market of its own (big money market that is).

Developing a good 3d engine from scratch is not a small task and despite of huge sums ready-made ones cost, it is often cheaper option.

Nevertheless, although a game uses bought-in engine, you can't talk about it as 'mod', maybe a distant hardly recognizable derivative, for the game that engine was originally build for.