ISO/MP3 combos mainly came into being because of file size. Back in the day, lots of people still had 56k or slower connections, and some people stupidly decided to rip the disc data, then make MP3s out of the audio tracks.
The problem with this is, MP3 at a decent file size gives you quality loss, some worse than others. Also the times on the tracks are also screwed up, and can completely break a game and keep it from playing in some cases, especially if the rip is done by an inexperienced person who doesn't know how to compensate for the timing issues, and likewise DOESN'T pass along any of this info to the people downloading the rip.
I opt for pure authentic rips, that contain just the CD image with the tracks already there, in redbook audio (standard CD audio) format.
That way you have as original as possible, a copy of the game.