Whether or not visually distinguishing a <readmore> block is desirable or effective is, I think, a matter of personal taste and style settings.

But as for why How (Not) To Ask A Question uses a <readmore> or why PerlMonks for the Absolute Beginner does not, there is actually no good reason to. Tutorials (of which those are two) never show <readmore> blocks collapsed as in other sections. The only possible effect is to give the block visual distinction (as is being argued in this thread). That being the only possible use, I would argue that one may as well use <table>s (or <div>s) to blockify one's writeup.