Why not crabgrass as a lawn?

Not certain yet(not certain that this is indeed "crabgrass") but looks like  crabgrass has flourished where the fescues have failed in my mystery grasses front lawn.  It is hell of thick and nice despite infrequent watering. No other "weed" dares grow through. Is this maligned "weed" perhaps the ideal lawncover?
Why is it so despised?  Im in coastal mid california dry summertime.
http://en.wikipedia.org/wiki/Digitaria
Hmm looks like it is an edible

I would guess that it would make a lousy lawn next year.  Since it is an annual.  And, since it is a warm season grass, and an annual, there is a good chance it will croak at the first frost.

Well, I'll enjoy it til then, its been a problematic area anyways (see my oxalia woes posts) and I may be incorrect on it being all crabgrass. have found good picks, but havent seen its skinny fingers..digits (it's Inflorescence, big word for me) as in the UC davis pics in link below.
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7456.html

if it grew in thick this year then I don't see why it wouldn't grow in thick next year

a major reason people tend to like the grasses they do for lawns is that they don't have to be mowed as often as "weeds".

There a sort of grass in europe called couch grass, that grows from runners. It sends out horizontal stems and new roots come from these horizontal stems and a new stalk and if you want t have a flower bed or a vegetable bed it gets into these and is absolutely horrible difficult to pull out. That is a reason to mind about some grasses.
  i know a gras that grows in the country here called rompe sacos, break sacks. It is horrible, the seed head sticks in you. THere is one sort of barbed grass head that can get up your dogs nose if he eats it and cuts up his nasal passages acording to a once freind o fmine. there are some unfreindly grasses around. agri rose macskie.

couch grass- would have to be the worst plant life on earth, i reckon even agent orange wouldn't dent it