It would be interesting to import music into GT5. But don't be like "how hard is it to get this done," sounding like PD can implement such a feature but refuse to.
Such a thing would diminish the issue of wondering what licensed music will be implemented in Gran Turismo 5. This would be the primary argument for this issue. It would mean that no matter what PD would put in for music, we already have our "Plan B" for whatever music we want to put in. Another argument one can make is that some people can make their own music and hopefully be able to port their own music to the PS3 if this were the case. If you could port your own MP3s into the PS3 while playing GT5, you'd have a music listing you can be proud of and not have to worry about what do don't want to listen to.
The negative argument one would pose is that in terms of porting MP3s and such, some would complain about copyright issues, illegal file-sharing, that sort of thing. You know, the legal talk about people porting in illegal music to the PS3 in listening to music while racing.
So where do I stand on this? One half of me says it's a good idea, the other half of me says that it's a potentially rejectable idea. So I'm leaning slightly more this being a good idea to make a long story short. But, if the positives outweigh the negatives, you'd agree to having this feature implemented into GT5, wouldn't you?
great idea! 
