Using the full address for links on mudcat was more of a problem in the past than now.
When there were multiple servers, using the full address in a link to a thread usually would take you to the server on which the original link was posted. If that wasn't the one you were on when you clicked, it meant backing out and reconnecting. If the other server was down, it meant losing the connection completely.
A local link would take you to the thread, on whichever server you were currently on.
Since we really only have one server now, it shouldn't cause a real problem to use full links now, although it may be a minor traffic hit for the 'cat. (Someone who knows how the 'cat server works may have an opinion of another sort.)
Note that most people do use local links within mudcat, so you usually can't "copy" a link to post on some other website.