This is a bad solution to taxation. It brakes the long-established tax practice of "realization principle". Suppose the same principle was applied to a home owner. At the end of each year your property is evaluated and you're taxed on the difference between last and this years price. You own an asset and this asset is valued by the rating agency as more expensive than before. Now you have a liability that you need to…
It made more sense in the past, when speculative valuations were less common and capital flight was less of an issue. It was collected from wealthy people, who could probably afford to pay. And it served as an additional incentive to invest your wealth productively. But then globalization arrived and made the drawbacks bigger than the benefits.