The big problem that I have seen with ignoring a member is that if you start reading a thread where they have posted in you miss a lot of information on what he is saying and you get into the middle of a conversation.
It's a lot like walking into a meeting that has been going on for a couple of hours, without all the information you are lost and might as well just turn around and leave.
However, I use the ignore function on this forum just to clean up the spammers post when they have posted multiple post. You place them on ignore, refresh the page and they are gone along with all of their junk.
I do go back into my ignore list a while later and unignore them since when a mod bans them they are gone forever.