There is also a important lesson which goes like this: your scripts should operate on a buffer (even a temporary one) and almost never on strings. Its generally easier that way because there is so much support from all the functions already exposed to editing tasks.
Hmmm... I think I disagree with this as well... strings are very very well supported... it's a little annoying that we don't have a full string lib in elisp (some of the things here http://emacswiki.org/emacs/ElispCookbook#toc1 are missing, but that's why I say you can copy and paste when you need to).
it depends on the context... but maybe replace-regexp-in-string would be the right function to use for strings in this context.
Of course, a lot of the time buffers are really powerful tools to replace strings. I just don't think your original assertion of "almost never" is right. Buffers are handy but I use them to replace strings less than 50% of the time.
