What's the effect of not installing torsion bar grease boots?

Long term the effect would just be dirt and road junk finding its way into the rear torsion bar sockets. The fair amount of grease in the sockets should mean that rust isn't an issue, but that grease of course is a magnet for dirt and grime. So you'd end up with a dirty, abrasive grease paste in the sockets that will probably hinder torsion bar removal/installation, getting worse as time goes by.

Not really an immediate problem, but it would definitely be better to have the boots in place for the long term. It would be more important if you drove in worse conditions. Certainly not all torsion bar applications use them, but they also have different socket arrangements.