The one mystery of decompressions is why don't the pilots get an auto drop down O2 mask (just like the passengers) but have to 'work out' that they might need one (from a noisey alarm and a big red light) and then put them on manually?
If you're dealing with an incident and have a high workload, the last thing you want is to have the mask suddenly drop down in front of you and obscure your vision. If your first sign of a problem is loss of cabin pressure then it might not matter, but if you're dealing with other issues and the cabin pressure reaches critical a few minutes into the event then you might not appreciate it.