Wonders are definitely a reason to keep a city but I don't see luxuries in the same way. If I raze and rebuild a city, I lose the wonder, but not the luxury. If the luxury is just outside the new city's first ring of hexes, you can always buy it - just make sure you have a settler standing by to rebuild the turn it turns to ash.
Apart from wonders, the only other reason to keep a city is if has very large borders and you don't want to lose that area to a neighouring civ while the city is rebuilding.
Small cities grow quickly, though, especially if you have a maritime ally, so you can burn and rebuild a 5-6 pop city in relatively little time.