i'm going to chime in here because i think about those JCB sidebooms all the time.
I looked at the jcb trackloader before i invested in the cat 257b in '05. My thinking at the time was why would i pay $55k for a jcb, when i can pay $55k for a cat and a cat will always be a cat. Also, if i recall at the time, the controls were mechanical, as were most at the time. The cat had pilot controls, and, well, nuf said.
HOWEVER, on the farm you do very little, ever, that requires you to be in the seat for more than a few minutes at a time. Mine is mostly a farmyard machine, and it gets used for lifting and moving stuff all over the place. Rearranging sheds, craning, moving pallets, handling deadstock, and the like. I find i want out of the cab quite often when the boom is in the air. Lots of times the boom crossmember is blocking the doorway, i have to climb out around it, either under or over.
I know it's unsafe, and that is my point. There are many many times when that side door would be a huge convenience and safety feature for me.