Why less when you can have more? (Pagers on *Nix)

Originally more only allowed forward motion in the file while less allowed you to move freely in the viewed file.

Modern version of more is more versatile.

I think the two wiki pages for more and less shows the current difference quite well.

Because less is more than more...

less does not have to read the entire input file before starting, so with large input files it starts up faster than text editors like vi (1). Less uses termcap (or terminfo on some systems), so it can run on a variety of terminals. There is even limited support for hardcopy terminals. (On a hardcopy terminal, lines which should be printed at the top of the screen are prefixed with a caret.)

"Most programs-- Ruby commands, for instance", should be using whatever pager is the default instead of explicitly using a particular one. You can set your preferred pager via the $PAGER environment variable, and any properly behaving program will use that pager. I recommend checking out most as well. view can be nice if you are a fan of vi.

Does anyone know of a pager that has emacs keybindings out of the box (or, even better, a way to use emacsclient as a pager that accepts data from stdin)?

You should be aware to the most pager, adding the capability of viewing multiple files simultaneously.