Ahhh - but tempo as a part of the recording process? Completely irrelevant. 1) Record piece 2) Record stuff that goes with it 3) arrange, discover that you're nowhere near any sort of reasonable tempo 4A) Adjust the tempo map to fit your drunken playing 4B) timestretch your playing to match a reasonable tempo map 4C) get the arrangement right then re-record your drunken tempo part so that it's no longer drunken 4D) Any and all of the above
Only ever make it to 4C, then look at the profusion of noodles I threw on top of original drunken line, imagine having to re-record them all to fit the tempo map and then spend the rest of the night/life weeping silently in front of video games instead. But I'm willing to grant that not everybody adds about 18 lines too many to every song, and if you're reasonable in your songwriting, this is a totally viable option.