The cab should not be a crumple zone.
The biggest danger putting a cage in anything, especially a street-driven rig, is not having enough clearance between your body (especially your head) and the cage. Retractable 3 point seat belts and most factory-style seats allow for a lot of movement in a bad accident which makes it really hard to have enough clearance to the bars. Good bolstered seats and harnesses are a very good idea if you are going to put a cage in your rig, as well as proper padding on any tubes that could make contact with your body.
An exo-cage is a pretty good way to go in vehicles with limited cab space; the main draw backs are that it is hard to get good triangulation around the doors and windows (if you want to be able to replace glass), they add width and hang up points, and they generally look ugly
Modern vehicles have to pass roof crush testing, but older ones didn't, and I wouldn't depend on the roof to hold up on those to even a single roll.