The Beatles were songcraftsmen, they sucked at/saw no value in improvisations, preferring a very controlled studio environment, which is perfectly ok. With Cream it was more the other way around (though they - especially Bruce - were still better songwriters than the Beatles were improvisers), which is fine too, nothing wrong with being a jam band. Horses for courses, some peole like to "
hear the songs they know and how they know them" and I'm more the type who grows listless if after the third song in a set a band still hasn't deviated from the studio versions.
I once left a Rush gig early (Permanent Waves tour) because they bored me stiff recreating their studio sound and arrangements - for all the complexity and creativity of their music, there was zilch improvisation, just utter concentration on stage. Girlschool were the opening act and while nobody in Girlschool would be good enough to even be a stage technician with the Rush crew, they sounded fresh, unperturbed and raunchy in comparison. Not that there was much improvisation
, but they weren't obsessed with reproducing their studio sound at the expense of a performance either.
To the defense of Rush, I have to say that their stiff earnestness in performing their music left them as their career progressed. I saw them 30 years later again and it was a fun experience.
And the late Kelly Johnson of Girlschool was as close as you could ever reasonably get to hearing Farrah Fawcett play lead guitar in a hard rock setting - what's not to like?!