Stock Airbox Lid, Good or Bad

Of the two bike I have (77 & 78 Kz1000's). When I bought them neither of them had a lid on the airbox. What's up? Are these easy to break or were the previous owners too lazy to put them back on?. Am thinking of ordering new ones,but if they break easy. What's the point? Any thoughts?
:S

If I recall correctly, I think they didn't attach well or the 'hinges' didn't work well. So owners did, in fact, get lazy...by either not affixing the lids well or leaving them off. The bike was designed to run with the lid on, so I'd replace it.

I still have mine but it has a spot on it where it melted down a bit. Not sure what caused it but I still have it on.