Biggest pro for car speakers is the nice mounting. It's easy to screw the speakers into the cab, very nice covers are available and you get a decent frequency range at earlevel (50-20000Hz). For best sound it is nice to make a stiff closed box of MDF behind the mounting hole.
Problem with 2.1 sets:
They are hard to mount. If you decase the sattelites, you have some flimsy speakers that are tougher to mount in a nice way than car speakers. And you loose the sound chamber of the case. So you need to make a new box around them to keep the good sound. Other option is too mount them unaltered, but then you have the chance that the complete speaker will rattle if not mounted very solid.
Last problem with 2.1 sets is the woofer. First I don't like to make another hole in a cab at the bottom. Second is that a 2.1 woofer is not a sub-woofer as with home-cinema sets, but a normal woofer, thus responsible for a very important part of the frequency range which is localizable by your ears (sub-woofer frequencies are less dependant on the location of speakers producing them). So if you play a cab, you hear the basses come from the bottom.