[sigzero]

Sure if you control the environment and can install Ruby. grep/sed/awk are always installed.

[dragonwriter]

Presumably, if you write ruby all day, there is a good chance that the environments you encounter have ruby installed, as well (and they might be even be Windows environments, in which case grep/sed/awk are not "always installed.")

[omarqureshi]

Portability becomes a problem though. GNU vs BSD sed/grep :(

[adrusi]

They do have a shared subset of features defined by POSIX. It's limiting and takes discipline, but it's not really impractical.