It should also be stressed that a "good" set of monitors are speakers that allow you to hear what you need to hear to create a good mix.
Monitors do not sound "good". I highly doubt that any monitor is truely "flat" either. Monitors do help you hear, however. A good monitor can be like a magnifying glass. A good monitor can make subtle problems that you might not always hear into huge blaring problems that you'll want to fix.
When I first got monitors I, like you, was unconvinced that they were necessary. I invested in a pair of cheap Event 20/20's and the second I plugged them in, oh man, I could hear! Suddenly there were reverb trails, drum squeaks, snare rattles, humming amplifiers, sloppy edits, etc etc!! I pulled up one of my mixes that seemed to play pretty well on my stereo and in the car, and holy smackers it sounded like garbage. Immediately there were dozens of things that needed fixing, and I didn't have to "squint" my ears to hear them. While I didn't hear these obvious problems when playing one of my mixes in the car, for instance, after fixing the problems the mix sounded BETTER in the car.
I also learned that the 20/20 doesn't have a lot of low end, so at first I did have a tendancy to over-do it (and perhaps still do at times). But what I've found now after learning these monitors is that they really force me to try to give the low end definition, which has slowly but surely been improving my mixes. I'm sure that every set of monitors has to be learned...as does every room.
At that time I didn't have a good power amp either. I was just using an old NAD integrated amplifier - underpowered and fairly colored, but a good sounding amp for home stereo use. Recently though I purchased a cheap Hafler TA1600 to power the 20/20's, and I was worried that I really wouldn't be able to hear the difference....man was I wrong. Suddenly the stereo field was THERE. I didn't hear "left monitor" "right monitor" I heard SPACE....vocals that really appear to come from straight ahead, guitars that off center and a little back, etc etc. Even my wife commented on the phenomenon without any coaching.
So don't be overly skeptical. You'll find monitors in every single studio in the world, and for good reason. I think it's a good thing to start on something like a home stereo so that you can appreciate the difference as you upgrade equipment....but someday you will own some monitors, trust me
My limted, amature opinion.
Oh, and I just re-read your original post. The reason you can't just go listen to a mix in the car, then run back into the house to make changes is....well...there are perhaps infinite reasons
You might listen to your mix in a car and think, "man, too much lead guitar", so you go back home and you turn the guitar - which you thought sounded fine - down. But then you plop the mix into your home stereo and "aw man, there's not enough lead guitar!" So what is your problem? EQ, compression? Probably a mixture of the two...but you can't really hear what you need to do while your mixing, so you end up buring 1000 CD's running around listening to mixes in various locations and trying to make good decisions. Then you run into the brick wall. *Every time you adjust one sound in a mix, the rest of the mix sounds differently!* It's a MIX. What happens if you take some of the sweet out of your sweet & sour sauce? You've got sour sauce. Haha.
Slackmaster 2000