They are made to be loud and only loud, way louder than the midbass's 100db +, and often dont extend high enough due to the high mass, high efficiency and PA/ Concert design.
They often sound harsh which alot of people on here dont like. If you go on a donk forum you will see them everywhere. I myself like the sound to be more smooth and flat, I dont like a boom and hiss system.
Some people do use them... I have seen them in cars before.
Main problem is a sensitivity issue... they are usually around 100db spl @ 1 watt and are designed to be matched with umpteen mid/bass drivers for live stage/PA/concert use. Hard to fit enough mids and subs in a car to keep up and still have good speaker placement in the confines of a car for good sound stage and imaging. Also said to have higher distortion. Plus, they don't come in a component set with matching mid and crossover. Flat frequency response is one of the most important things.