I have a question - if the sponsor is eligible to sponsor a spouse (inland application) without an income, why has he/she bring to the landing interview a current letter of employment?
May be to show them that sponsor is able to take care of the sponsored person. Mind you that income requirements, and be able to support the person to be sponsored are two different things.
If you are sponsoring a spouse (of course there are no minimum income requirement) but at least IO wants to see that money (doesn't matter how much) is coming from some source of income and that will show IO that sponsor is able to support the spouse. This is what a sponsor signs while sponsoring a spouse. The IO may wants to make sure that the sponsor will not go on welfare right after the spouse gets the PR.