I have built one and having done it I can definitely see room for improvement. It's far more secure than it looks. You can turn it and not fear your pipes flying off as long as you don't spin it like a tire and keep your pipes out of the edge. Definitely not something to let the kids play with.
To build another I think that some manner of appropriately stiff material in the slots that would envelop the stems when they were pressed in would improve it. Maybe slightly over-sized cork or felt, maybe some kind of rubber. It would also improve the grip a bit. Otherwise I think it's great. Very attractive even when partially filled and a great display for pipes. I do not think that your stems are in danger by being in there as acrylic, lucite, and vulcanite are pretty robust materials and as far as bending in there...well the possibility is there but I'm positive that they would have to sit there undisturbed for years before you saw any change.