Sometimes, you can actually come out better with home speakers in cars, than car speakers in cars. IMO, there are two main disadvantage of using home speakers in car.
1. There are no 6x9's and 5x8's in home speakers.
2. You will get less loudness using the same amps.
That's about it.
#2 can be an advantage too, because it is related to the impedence and the damping factor. You stick home speakers to a car amp, you get twice damping factor than car speakers in a car amp. More impedence, Less load, cooler running amp, more control of cone movement etc etc etc.
Well. Everyone's entitled to their own opinions, but there are lot more than what the manufacturer says on their box and advertising materials (including magazine articles).
I am putting these home speakers in my car.
My tweeter (HIVI research TN28 $20 each)
My 6.5" Midrange (HIVI research M6N $45 each)
My 12" Subwoofer (Dayton Audio 12" $130)
With active Xover/GEQ/PEQ/Delay using SB0350 with KX project.