I don't know how holding will help you. But decompressions happen suddenly and you feel as usual. You are not getting prePared for holding your breath.
Also remember that pressure is droping fast, if you hold your breath you'll have barotraumatism at your ears surely and maybe at your lungs, it happens for scubadivers if they hold the breath while surfacing.
Replying to the other comment, the first thing that you notice is you ears poping, maybe some discomfort at the sinus and finally you'll see the oxigen masks drop when the cabin altitude reaches 14500 feet. You wont feel how you are losing your cognitive capabilities till you pass out. Check the video I posted above to see how it works.
If you've been told to hold your breath, sure. If the pilot quietly drops the O2 level in the cabin, what % of people would comprehend the situation in time to get a deep breath?
Can't most people hold their breath a lot longer than that? 30 sec seems like a minimum for the average person.