If you don't have a separate office, then odds are you have a sub-optimal work environment. So you're not considering "if you can save money working from home" but rather "if you can save money by working from a worse work environment". Which I guess is a valid argument, it's just somewhat orthogonal to the question of whether or not working from home saves money.

> only a desk of space wasted

There is a lot more to a dedicated, optimal work environment than "only a desk of space". Plus the fact that a LOT of people live in places that don't have an extra "desk of space". Approximately half the places I've worked couldn't give up a desk's worth of space without it being in everyone's way.