i personally dont get why. i am using the evods right now and they are pretty finicky. the heads lose juice all the time and you constantly have to adjust voltages to try and clean out the gurgle. you have to 'sip' on it and i like hittin it like a cigarette.
Just add a small piece of wick to reduce the juice flow. Nature of the beast with gravity fed clearo's. juice viscosity, and air-flo differences have to be balanced by how much juice can pass to the wick. At least the EVOD and Pro Tank head design makes any adjustments that may be needed easy to do.