There are several problems with underlining, described briefly at TextEmphasis.

* Underlining is in context of the Web associated with links. Sure, one can style links differently on er site, but e can also do it with <em> or <strong> as well -- turning them into underline;
* Two ways of marking emphasis are enough -- one way would be sufficinet if not the need for highlighting/not-highlighting distiction;
* Underline has the same meaning as italics (see http://www.c2.com/cgi/wiki?RealMenDoNotUnderline);
* WikiCreole is supposed to be a "common part" of various wiki markups -- many wiki engines don't include underline (for the reasons mentioned above);
* Creole is extensible -- if a site needs certain markup, they are free to add it -- and it's not breaking Creole;
* What next? Blink? Marquee? Sparks? Zooming text?

Are there any use cases where underline is really required?
Because adding things to Creole makes it more complicated and harder to learn -- so if an addition doesn't really improve the markup -- it's better to refrain from it.

-- [[RadomirDopieralski]], 2006-12-15

Underlining was originally designed to replace italics in the case italics were not available.  I really don't see much point in it.  It's rarely used on the web anyway (except in links, and those we already have).

-- JanneJalkanen

-1 on underlinig. +1 to all arguments above! -- [[J├ÂrgGottschling]]