I'd agree that playing 3C when you're a pool player can mess you up a bit. Faster Table, Larger Balls, and all the rest. But that being said exposure to the game for B players and below has lots of benefits.I think you are making an assumption. I would say Efren probably played 3-c from the start of his playing history. Although billiards may have some learning that can cross over with pool, the biggest problem is the stroke. If you get on the average billiard table, not say one of the heated super speedy tables, and then try to go back to a pool table your game will be a bit messed up. If you are playing a lot of serious pool it may be a good idea to stay away from billiards.
Something interesting, pool players seem to have less problem playing billiards then billiards do playing pool. My wife used to beat Sang Lee playing 9-ball. It was amazing how bad he played pool. Although I have to add that Blumdhal plays jam up 9 ball, he even beat Efren in an exhibition. I didn't know how good Blumdhal played. I was practicing 9 ball and he asked if I wanted to play some. I just assumed he meant Billiards and started to walk over to the billiards table. Who in their right mind would not want to play Blumdahl billiards. He said know lets play pool. I was shocked how good he played.
Learning natural tracks of the CB: I've noticed beginners having a hard time visualizing, for example a 4 rail shot Short,Long,Short, Long. While at the same time have no problem seeing the same shot only banking an OB.
Learning how to spin the CB and how much. IMO most pool shots can be and maybe are made on the vertical axis with various speeds. There's not much occasion to load up the CB with side so pool players in general don't have experience in those areas of the CB. If you tell them to get out there 3 tip many will think you're putting them in a miss cue zone.
Your comment about Sang Lee makes me feel a whole lot better. I'm probably a C- pool player. Maybe worse. CB goes where I want, OB goes bouncing around the table. Not a winning formula.