The relatively short telescoping shaft with a U joint on both ends will have a bad habit of binding at certain angles, particularly if oddball terrain is encountered. The movement of the axle, the springs, and maybe a little frame flex added in can cause the steering column shaft to get into an angular bind sufficient to lock the steering wheel at a deadly dangerous moment. Since the rack and axle move as a unit, bump steer won't be a problem, but the geometry of the telescoping shaft will be unpredictable and dangerous.
BTW, I have run telescoping PTO shafts on my farm equipment for 30-something years, and your argument comparing them to a steering column shaft is totally bogus. There is virtually no comparison since the operating angles are very shallow on PTO equipment and the shaft length is many times what a steering pinion shaft would be.
Jerry
