It's pretty much that!
If you're making a commercial game, it's best to develop your own original style because it helps identify your game as your own, and not some RTP-styled game. It's definitely not wrong to use the RTP-style, albeit, as mentioned before, you can't explicitly use the RTP resources as per Enterbrain's EULA.
If I were playing a commercial game with RTP graphics, I'd initially be thrown off. It would have to have really awesome gameplay, story mechanics, or some other vital field to let me overlook that. Though... The Aveyond series seemed to have enjoyed a niche group of people, and that's RTP-styled and commercial.