look, most people here are making valid points. atomsmasher, the one time i bought a new car was for my business. and like you say you want that extra bit of reliability (i was a courier, i especially needed that). i put 150,000km on it before i sold it (daewoo matiz, thats a lot for an 800cc car). near the end of me owning the business it started to play up. first the water pump, then the clutch. then an aircon compressor bracket. quite acceptable though for a work vehicle. if i had continued, that would have been the time to buy another car (after 3 years in this case). If you are a very well paid person of some importance to the company you might want a new car too, because you dont want to be late for meetings, appointments etc.
for all us 9-5ers, if you are late every now and then for work because of a car break down, thats quite acceptable. if the boss were to fire you for that, who wanted to work for an a-hole like that anyway?
patrickl, i still dont follow your maths. especially with the luxurious position of replacing new cars every 4 years. lets break it down over four years first (ball park figures to get a handle on whats happening) in dollars. the cars in this case are similar in fuel consumption so we need only look at extra costs:
new car old car
mid sized small mid sized small
year one 30,000 15000 5000 2,500
year two 0 0 500 500
year three 0 0 500 500
year four 500 500 1000 500
total 30500 15500 7000 4,000
thats the kinda 30% id like my shares to earn

ok, now to year five
first we sell our cars. this is what we get for them:
new car old car
sell 15000 7000 2500 1000
so this is what we have left after four years. in four years we have spent 15500/8500 on a new car, and 4500/3000 for an old car. now to buy another car
year five 35000 17000 7000 4000
using the money from the previous sale, we have spent another 20k/10k on the new, and another 4.5k/3k on the old. the total spent so far by year 5 in this simpistic model is pretty much the bottom line- new 35000/17000, old 7000/4000.
a pretty simple example i must admit. it doesnt include the fact that your old car might suffer such expensive wear (like replacing an engine) that you are better off just buying another car. therefore just double the price on the bottom line for old cars from 7000/4000 to 14000/8000. thats still less than half what you spent on the new car.
thats one extreme, the other is what my dad used to do. he would buy really beat up old cars (even from the wreckers, and at least once was given a car) and drive them for a year or two (and fixing them with parts scavenged/wreckers) before buying another one. make that bottom line $500 x 5 years/$400 x 5 years= $2500/$2000! following his lead,my first car was $150, spent maybe $200 getting it on the road, and then sold it for $800 a couple of years later

a cortina mkII. sigh, the memories...
and yes, you can get mid sized cars for 20k or so. the quality will be a little less, and more chance of things going wrong
year one 20000
year two 0
year three 0
year four 1000
year five sell, get 10000
buy new car 22000. only if you compare a new small car to an old large car will the figure get closer, in my opinion...
this is an economic reason only. yes, it is freakin cool buying a new car and being the first to own it, and having smell new for several years (several months in the case of a couriers car

). but for some of us, its even more cool to have an extra 10 or 20 grand to play with in 5 years. not many of us (including me until the last couple of years) will be able to show you 20 grand after 5 years, but it would have meant a lot more comfort and the ability to buy other new things! even i wouldnt buy something like a TV or console second hand if i can get it new. hell, i even spent $90 on a sega genesis lately because it was brand new!
edit: when i say economic reason, obviously im talking about personal economics. on the national or global scale, people need to buy new cars to keep the economy going.