To each his own. I spent a lot of time in the car, and loved music. Had a 2 hour commute to work (39 miles, dang rush hour traffic was horrific).
Unnecessary weight? It's only the kids and their boom boom music that the sound systems weigh as much as the car. Put ten 15 inch subs in the back with the enclosure made out of 3/4 inch MDF and it's bound to get heavy.
The only thing added to mine was 2 small amps, a very small pre-amp EQ, and a small enclosure for two 12 inch subs. The sub enclosure was the heavy item, but probably at only 30 pounds or so. That was replaced with 2 smaller enclosures for 10 inch subs. Very small and very light.
Radio? Well, I took the junk factory out and replaced it. Front speakers? Yes, I had 4 speakers up front. But 2 replaced the door speakers and the other 2 were even smaller than the factory rear. The factory rears I removed, I ran nothing in the rear.
For what you pay extra for the premium factory sound systems, I could probably get you something that sounds 100x better, yet only cost you a couple hundred dollars (US).
Bob, just using the integrated amp on even an aftermarket head unit isn't going to do a whole lot. Yes, it would be better than a factory unit, and any new speakers will be better than stock. But don't go nuts over the speakers. Just about anything from a reputable manufacturer will do, shop by price.
My equipment of choice is Infintiy and MBQuart drivers. Polk audio is good as well, but I've never had a pair. For subwoofer, can't say because I've been out of it too long. My subs are still the same 15 year old 10-inch MTX Blue Thunders. Actually, all my equipment is from back then 15 years ago.
Amps, my old Kenwood was the cleanest sounding amp I had ever heard. Even though it was only 16watt x 4, it far out performed every other system I had heard for normal music (anything but rap is normal music.) I also absolutely loved my Denon DCA-500 50Wx2 for driving the subs. I didn't play rap, but the bass in "normal" music hit hard and hit low. As I said last year, I could put Rush's YYZ drum solo in and no one with their booming systems could touch mine.
The only thing I shy away from now is anything from Sony. They were close to top dog at one point, but quality has gone down hill. I've never seen so many problems with audio equipment as I've seen with Sony. That said, I do have the very first in-trunk 10 disc CD changer that came out, the Sony can't remember the model. Bought it in 1991 before the downhill of Sony. I still have it, it's been through 6 vehicles, I had to tear it completely apart to remove a CD after a vehicle accident ejected the magazine, and it still goes. I haven't cleaned it in probably 10 years, and I could still go 4wheeling and have it never skip. Since then, every CD player I've had from Sony, car, home, or personal, has broken.