I would say the single most useful thing in bash is the ability to type in commands.
I guess I re-run commands fairly often, so for me the history-search-backward thing is very useful; when I just need to use the default up/down binding, I just hit Ctrl-E Ctrl-U (go to end of line, erase everything to beginning of line) and I'm good to go.
Ok, but what if your prompt is empty and you press up once? Will it still continue to maintain the old behaviour?
To expand on this, it searches based on what's on the left side of your cursor. So if your cursor is all the way on the left side, it acts as it did before. You lose nothing!
I prefer to map it to esc-p which coincides with the setting in tcsh. Like that:
"\M-p": history-search-backward
So you basically type something and then press esc p to have the shell complete it.