Pro Audio speakers are for getting loud with little regard to clear sound. They're meant for DJs and such who will run them damn near fullrange and need to be able to fill a large area with sound, where nobody will be listening for perfect sound reproduction, or even good sound reproduction. A lot of times they don't sound very good up close either.
As with all things, you can throw money at it to fix issues with it. Big concerts have VERY expensive equipment they use to get loud and still maintain some SQ, but your average PA speaker is designed for small time DJs that do little parties and stuff.
Basically, if your music is for you & your passengers in the car, PA speakers are just going to sound like crap. If your music is for people outside of the car, PA speakers will get you heard from farther away.