For me personally, I'm 6' tall with orangutan arms and prefer a long cue/bridge.
I shoot with a Jacoby cue and shaft and had them both made to 31" - then added a 6" extension in the middle.
I have a tried a couple of butt extensions before, but prefer the shaft extension.
That's me though.
Logic wold say that a cue extension added to the bottom of a cue butt is for reach only- a long bridge substitute and an extension added between the butt and shaft would be for continuous play with that cue.
For me personally, I'm 6' tall with orangutan arms and prefer a long cue/bridge....