I've helped build and upgrade a lot of rifles over the last few years. Often, someone will give me the parts they're removing to upgrade. Late last year I looked in my box of goodies and discovered I was exactly one stripped lower and one BCG short of a complete rifle. I don't have many spare parts now...
But really, I see it as a matter of need or convenience. If you rely on the rifle, you need some sort of backup. Often, it is cheaper to have a few spare parts laying around than to have a complete backup rifle. If it's just a range toy, no real need to have spares. But even for the expense of going to a class, having spares can mean the difference between finishing a course you paid for or sitting it out and probably not getting any of the money back you laid out to attend.