I'm a great believer that the highly paid trained engineers and designers at top car companies generally know best.
Yep, they do, then people go and ruin the time they spent and the millions the manufacture invested by changing the shockers, springs, wheels and tyres
Check this out, a lowered Ford GT
http://retrorides.proboards.com/index.c ... 973&page=1
In answer to your question Velo in most cases on a road car it's mainly for looks, most cars (saloons and hatchbacks included) look better closer to the ground, when i've lowered cars i've aimed for having the top of the tyre an inch from the arch lip when viewed from the side, then the wheel hides the normaly manky inner arch area.
As mentioned the limit for me is the bottom suspension arm, if the car is lowered so much that the arm has got past level to the ground and has started to point upwards the car will be terrible to drive.
It is a compromise between looks and driveability though, i could'nt live with the Polo that low everyday, drove ok but would crash into pot holes instead of riding over them, also speed humps were a no go, maybe as a weekend toy but was a PITA as my only transport so i changed it back to sensible, i have had lots of lowered cars in the past, i'm getting old now though and like my comfort.
There are plenty who take it to extremes though and will have a car scraping along the ground or be hellish to drive.
Lower is'nt always best though, even for race cars, check out this awesome tarmac spec Metro 6R4, riding quite high.