I'm in two minds about a roll cage.
Pro's - Safety in the event of an accident, added stiffness to the chassis, somewhere decent to attach your harnesses
Cons - Cost, weight, danger to your head if you drive the car on the road without a helmet, pain in the a*se to get in and out of the car.
A few other questions would also need to be answered:
Does the cage make the car safer in an accident if it doesn't roll?
Can you get out quick enough in an accident with a fire?
Does the extra chassis stiffness counteract the extra weight?
I would probably go for one if I had the spare cash, knew someone I trusted to install it, and was made of rubber to aid getting in / out.
I've waited nearly 3 years so will probably leave it until I can find a cheap one.
Paul