In our cars they really aren't necessary. The oem system comes with a crossover network meaning the oem speakers are already protected from being asked to reproduce any harmful frequencies. And if you upgrade your system with aftermarket speakers and an aftermarket amp, your new amp will most likely have a hi and low pass crosssover to do the same. So no, you really shouldn't need a passive crossover (bass blockers) with our cars.
Now if you had an aftermarket deck, and bypassed the oem amp, they might come in handy. Especially if you added a sub as you wouldn't be able to hear the mids distorting as easily.