It IS just a wee bit less taxing on the server if you take them off your ignore list. See, if someone is on your ignore list, each time the server displays a thread for you, it as to check each message for whether or not the poster is on your ignore list and react accordingly if they are (by removing the text of the message). If your ignore list has people in it that have been banned, the server still has to check if those posters are in the thread. Removing banned users makes the server’s work just a little bit easier, and every little bit helps.
It’s of more use on boards where a greater diversity of posted content is permitted, particularly HTML (including flash embedding) and images; if there’s a particular poster who habitually posts huge, pointless images, or flashing animations, you’d be able to avoid seeing them (and the thread would load into your borwser more quickly too). We don’t have that problem here because of the limitations of content (and the standards of moderation) here.
The other major use for ignore lists is to be able to annoy people you don’t like by saying “I’m putting you on ignore now!”. But that’s against the rules here (and rightly so, IMO).
Excellent point. I hadn’t thought in that direction.