no prob 
there's 2 reasons to fitting a rollcage,
a) for protection. as jonathan has pointed out, it prevents the
chassis from getting crumpled in and crushing the occupants in the
event of a serious crash.
b) for additional chassis rigidity. the structure of the roll cage
reinforces the chassis and prevents the chassis from flexing during
hard acceleration, braking and cornering. this aids greatly in
improving the overall handling of a car.
u dun find road cars installed with roll cages simply coz its
illegal. its illegal coz most roll cages can only be installed
after extensive modifications have been made to the existing
chassis to accomodate the roll cage. moreover, as the purpose of
the roll cage implies, its like telling everyone ur car is used for
racing. a taboo in local road terms.
