They're nice, but they must be practical. Building a 1km high skyscraper that is impractical just to get on the record books would be a disaster. It would have to actually work well for whatever function the building is for.
Mulholland's link said:The building, known as the X-Seed 4000, is designed to house up to one million residents on as many as 800 floors! Designers have had to consider tricky questions of temperature and pressure differentials between the base and topmost floors, and are looking to utilize solar power to solve these and other critical issues. The cost, you ask? Somewhere between $300 and $900 billion... what's that, an Iraq War or two? Couple of manned Mars missions? Quite do-able - if you're Japan, one of the world's richest countries.
But there'd be a whole host of issues regarding this, not least of which would be the building itself as outlined above. Dumping up to a million residents there would also affect services, traffic, and just the speed of which a resident could get out of the building to head elsewhere.