Seanf talked about using a side fence on his hand held router, and that is the kind of set up I was responding to; maybe I should have made that clearer, so sorry about that. Extraction in hand held router use isn't attached to the side fence, or at least I haven't ever come across that arrangement.I will usually use a fence on my router table unless I need to cut a curve, when the fence can’t be used, as the fence has dust collection in it. I’m currently using a slot cutting bit with a fence. So superfluous? Not so much IMHO
In the case of inverted table mounted routers extraction via a fence is normal so, where it's practical, use of it for this purpose obviously makes sense. There are arrangements in addition to fence mounted extraction for for sucking out dust and so on usually involving hoods or similar that can be set up for most curved work. Slainte.