Think of it this way. You aren't providing your affiliate link to other shopkeepers. You are providing it to affiliates. Shopkeepers aren't affiliates. Those are two different hats.
Now a shopkeeper can also be an affiliate, and at that point it is their responsibility to know about the XID parameter. It's nice of you to include it, but an affiliate who is not a shopkeeper is going to look at it and ask "What is an 'XID'? I don't have any 'XID'!"
So, it is optional in that not all affiliates use it. Only affiliates who also have shops and may end up selling their own product use it.