Hi Lee,
Basically because I am a big guy with a bad back and don't want to bend over to mess with the nipples or a valve all the time to adjust the temp.
I also use an iQue-110 stoker and the draft doesn't matter as much. I have read that tall intakes don't draft as well to control the temps. I cannot speak to that, but I shortened the stack just in case.
Here is a later pic. I shortened the pipe and painted the UDS so it is not nearly as ugly. This pic shows the iQue hooked up.