At rehearsal im using monitor out with cab off into my peavey valveking and marshall 4x12 1960 cab and one main out to a yamaha dxr 10. Sometimes i dont bother to connect the amp and cab since its so directional, but it sounds good with both.
Playing live i just use my dxr as a monitor and feed main out to the mixer.
Guess you could stick a mike in front of the speaker and do it the old fashioned way, but theese days the venues and soundguys aren`t really overly fond of loud stage sound.
For me the best way is going direct and get the "same sound" as my dxr and have less unknown parameters changing your sound not regarding the different PA systems at venues though:P
So the reasoning regarding an amp and cab is more the feel and thump during rehearshal and more of an hassle when it comes to live gigging if you ask me:)