I understand the preference to have "action" on one's route, but personally, i don't use carz. i have two routes that i split my working time between (both will be released...at some point

) and i have carz disabled on both. on one (a U.S. themed route) i have it disabled because it makes the setup of ATLS-triggered crossing simpler (the car stopper wants to sink the road...at least the YARN type...about 2.5 meters in the ground. It's a pain to fix once it does that) and stops the annoying sounds, suicidal drivers, and bad-looking vehicles from clogging my routes. i don't have a powerful computer anyway, so it's only a benefit to me.
on the other, which is british themed, i have them disabled for different reasons. there are only 2 crossing on the route, but it is based off the 1950's and the modern cars aren't really appropriate. also, the route is based off a famous movie from the '50s (guess which one

), and one of the key scenes is a race between a bus and a branch line train, so i have invisible track in the road for the bus to "drive" on. as such, the cars would have interferred, with the bus plowing through them.
just my two cents, take it for what it's worth. my reasons are mostly developmental based, plus personal preferrence to keep my rustoleum reject computer from throwing up the white flag.