I agree with bookworm and Wino on this subject. If operating under FARs Part 121, it makes no difference if it's quick donning or not. If one pilot leaves his/her seat above FL250, the other puts on the mask.
Part 135 ops only require it above FL 350. The rules in other states may well be different.
Those who have experienced a rapid/explosive decompression (either in real life or a pressure chamber) are not so cavalier about this. You want it ON, not just handy.