I guess the rear half could be fully welded together (it will still fit in the car in one piece but it's awkward and requires the whole interior be stripped) then there would only be 4 removable joints at roof level.

Often those bolt together cages are designed to fit with no modification to the trim. ie; the dash and hoodlining remain. Therefore the fit of thye cage is compromised. I took some pics of really poor cages recently at High Country, one of them the driver could look over the top of the front leg of the cage it was that low! Another had the front legs and diagonal brace that far back that the driver had to step over the top of his seat to get his legs in. Either way, a removable cage will be a compromise in safety, strength and design, but I can understand the desire to keep a road car as well.