So the laptop battery goes awol in an unlocked bin at 38000ft. You still have an overhead bin fire to contend with. Who says the unlocked bin fire will be noticed and acted upon in time?
If an LiON battery goes nuts in a bin at 38000ft, I rather have that bin locked. First of all, you'll have a very hard time extinguishing the fire anyway. Second, let the smoke stay in the bin. Third, don't help the fire by adding oxygen. You'll have a lot more panic if you have a burning laptop in the isle.