I think if a truck hit you from behind in a mini. The cage is the last of your worries. May be picking semi trailers out of your a$$ for the next while

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1970 Cooper S ex-Bathurst & ATCC
1964 Austin Cooper S ex-Group C race car
1967 Morris Cooper S ex-Group B
1962 Mini Speed sports sedan
1968-71 ex-Peter Manton Shell car

Umm, a lot of minis have been hit up the bum over the years. It just depends on how hard it hits. But I can't see why you would want to add something that could cause more injury than not having it at all.
When you are in the drivers seat you have a fair amount of crumple zone behind you.

Oh, I used to make roll cages for race cars. I do know a bit about the subject.

Beware of bolt together cages, I know of an accident in a rally car where the bolt together cage came apart and killed both the driver and navigator - they become objects that will beat and/or impale you.

In my view, the only cages worth having are professionally built that tie into the body at multiple points.

If the cage was deemed to be too dangerous for street use then the use would be prohibited entirely. If you want a cage put it in. In my opinion the benefits of a cage far outweigh the disadvantages. I dont think anyone has the ability to generalise and comment on the risk involved without individual assessment. If anyone thinks they are qualified to please let me know. I will have you sign my next engineering certificate.

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1970 Cooper S ex-Bathurst & ATCC
1964 Austin Cooper S ex-Group C race car
1967 Morris Cooper S ex-Group B
1962 Mini Speed sports sedan
1968-71 ex-Peter Manton Shell car

As you were saying, it is all about "...too dangerous...", that being the level of risk. Different cages offer different levels of protection. Bolt together certified cages offer less protection than a certified welded cage - I can promise you that that is fact and is represented in motorsport around the world.
You can buy a car with an ANCAP rating of 3 which will offer less protection than another car with a rating of 5. Does that mean the car with a rating of 3 is dangerous? No! It still meets the Australian Standard but may not offer you the level of protection you think it might as a consumer.
I think we need to be mindful that rollcages were designed for motorsport and not road use and with the intention of the driver wearing a helmet and having a harness to prevent the occupants hitting the cage.
It would be easy for every manufacturer to improve safety in all cars by fitting rollcages ex-factory, but they don't because the negatives outway the benefits for road use and instead design goes into passenger cell technology.