Of course, the big issue with copyright is not actually protecting the author's work from somebody claiming it as their own (that's just the noble-sounding reason we give)...it's actually money.
The copyright issue as I understand it is this: Basically, it's only worthwhile for Sierra to pursue copyright infringement cases if they perceive that they are losing money because of the infringement. That isn't particularly likely for a few graphics stolen from their nearly 20-year-old games. Now, if somebody did a remake of one of their games that they still sell (for example, Leisure Suit Larry) or started distributing those games as freeware, then it would be in their interest to pursue the case. Otherwise, it would probably cost more than it would benefit them to press charges.
Where exactly it might benefit them in terms of stolen graphics is fuzzy. If you borrow Sonny Bonds from Police Quest 1 and use him as ego in your game, Sierra might be able to prosecute by saying that you're making another Police Quest. However, if you borrow a door, they might have a harder time doing anything about it.
The big issue is registered trademarks. Sierra HAS to contest registered trademark infringement, or they lose the trademark. I'm not sure what things are registered trademarks, but it might not be a good idea to make a game with Space Quest in the title.
However, it is, in general, better to use your own graphics whenever possible. It might speed development to steal graphics, but you might be putting yourself at risk.
There are modified Sierra graphics in the demo of Jen's Quest. The guards in front of the gate are roger wilco from SQ1 painted black (and with new animation added), and the car is a recolored car from Police Quest. I'll probably eliminate those in my next demo and replace them with stuff I created.