You would obviously have exceptions for renovations, subject to reasonable proof that renovations are actually taking place (or planning approval is being sought for said renovations). Ditto for make-safe work if the house is deemed unsafe to live in.

I can't really think of any other valid reasons.

As long as it’s cheaper to have somebody living in a house than just hoarding as much land as possible

Not necessarily. Some people just can't be bothered with the hassle of figuring out renting it out. I've heard this is especially common with Chinese buyers since housing in China is viewed even more as an asset than it is here.

If someone buys it as an asset just meant to appreciate in value, the dividends from renting it out may be viewed as less important.

It also prevents the use of a house as a rarely-used holiday home. If someone spends a handful of weeks per year in a house, that is pretty much just as wasteful as leaving it empty full-time, as far as the housing market is concerned. Maybe that levy could be decreased proportional to how much time it actually is being used.