A good roll cage may save your life, a bad one will kill you.
Roll cages in any case where you are not wearing a helmet are slightly stupid.
In the case of a crash, with your body flying about the place, a roll cage is just a very very hard place to hit off.
In most cases its either a case of going all the way or not at all.
But in some, such as the Porsche GT3 RS, it is acceptable.
Porsche justified fitting a rollcage for a few reasons.
1) It is rear only
2) There is a harness bar, so 3 or 4 point belts can be worn
3) The seat back is very large, so this will prevent any stray body parts from reaching back there in a crash.

I would never fit a roll cage into a daily driver but after all, there are allot of ways to stiffen up the chassis other than roll cages.
If you look at the Scoot rx7, koseki-san didn't feel the need to install a rollcage, but instead he strengthened the shell through seam welding, strategic braces and reinforcement plates along with filling the frame cavities.
Also, dont forget about the lockable 3 point seat belts.
Where you fit a new buckle that locks the lap belt in place, but allows you to move your top half, so if your reaching to get something, or folding out of the way in a rollover, it is a win win situation.
This is used allot in the stunt driving buisness