Assuming it's a capital repayment mortgage, you're neglecting that you are erasing debt, only the interest payment is really a cost here.

But yeah I think it makes sense to exclude it anyway, needs/wants might change and you no longer have the room(s) available to let or whatever, and it wouldn't really be a useful comparison of your 'side project' earnings to compare a year as a landlord with one not, it's a different thing.