Actually, the habitat allows for any ship to be built at industry instead of shipyard. Or at a shipyard if you manage to get one massive enough. You can make a MASSIVE warship if you stick an orbital habitat on, because then you're not limited by shipyard size, only by your maintenance and engine power. But, you'll also be lugging around the added weight and expense of the habitat...
Either way, it seems most people build their habitats without engines and keep them very commercial. They're useful for extremely hostile planets, as it allows you to occupy an N/A cost world, 50,000 colonists at a time. It essentially gives to 50k infrastructure-free population. On earth it's useless, unless you're building up a different species who can't survive on earth, then a habitat will let you keep a few in orbit without worrying about infrastructure. Or if you're wanting to put some non-auto mines on an asteroid and need some population to run the mines. Not to mention the roleplaying value, but that's another argument altogether.
