Im not too sure how the "ignore" functions work "under the hood" but I think the client must still have to do something with all the text within the channel? It cant just straight out "block" the text I dont think.....thats like various "Channel Filters" yea, itll make your chatting a lot easier, but it wont stop the bandwidth usage, because the text must be "received" before it can be "filtered".
Does this make sense?