Is it any different from C-a C-k ? Both seem to stuff things into the kill ring (or whatever it's called in bash).

I guess lots of people leave C-a as the default escape key for screen.

Yea, there's the GNU Screen mapping conflict, and also I rarely use C-k so it's not in my muscle memory.

Makes sense; I use emacs in addition to just the emacs-like readline bindings, so C-u conflicts with "prefix argument". I also nest screen sessions using ^Z as the outer one's escape key and ^O as the inner one's, partly so I can use C-a / ^A.