Anyone remember someone who was "gonna make you mah bitch"?
Romero licensed the engine, had a decent team and look what they put out? Daikatana.. ick, but Anachronix (whoever it's spelled)was actually quite enjoyable from the demo but not enough to get my money.
Companies like Raven save alot of money by using someone else's engine by not having to spend the time/money into building one from scratch. This is typically the longest part of game development. Plus Raven just makes damn good games.
This is why it surprised me when Valve decided to use a from scratch engine. It's not like Sierra isn't hurting enough for money as is, but they are offering the resources to build a new engine as well?
After playing Rainbow Six 3 on the Unreal engine you can keep your new fangled engine, I already like what I see elsewhere.
As for physics.. UT 2k3 shows how cool the Karma engine works with the Unreal engine, as well does the R6 3 engine.