The housing is a useful building - it houses the monsters you want to use for attacks or defending your base. You can choose to protect your housing or leave it in the open as part of the NEC. Few people protect their Housing, but it is actually a plausible idea. If there are monsters in the Housing , then
Walling up the Housing
The housing is not normally walled in because of its sheer size - it would take many blocks to fuly surround it, and due to its size it could mess up your sleek yard design and throw your planning balance off. Mostly box yards defend their Housing, but some queer (but not weak) yard can do it too:
This Yard is very interesting in the sense that it has no other resource generators other than Goo Factories, so the owner would have an easier time walling up everything else.
If you can find a way to protect your Housing (if that's what you want) and still have a nice yard, good for you! If you constantly get attacked while feeding your precious champion and want to protect your housings, no problem! If you don't want to lose all your Goo in one silly invasion on your yard and decide to wall up your Housing, ok! But personally I don't believe in protecting the Housings.
Housing as part of NEC, Shields
Housings are most commonly used in NECs. The housing makes D.A.V.E.s stand far away to attack it, successfully helping your NEC a great deal - the NEC simply wants to get the monsters as far away from the important stuff as long as possible.
Sometimes people don't want to defend their NEC to make the NEC further away from the Town Hall, but some prefer to have a "shield" of useless buildings around their towers and silos, so the Housing still plays a role. By forcing D.A.V.E.s to stand far away to shoot the Housing, the D.A.V.E.s will most likely not breach your shield of useless buildings so quickly, delaying them long enough to do just a little damage.
Credits: Nearby guy's yard above.