As you know, and probably a lot of other people on this site also, those clips keep the brake drums in place while the car is moving down the assembly line. The rotors and drums are put on earlier in the assembly process, but the tires are put on close to the end. Knowing union rules, if a brake drum would fall off halfway through the assembly line, the nearest guy wouldn't be allowed to reinstall it.
Once the clips are removed by the tech or customer, only someone with OCD who really wants to keep the originality of the car will put them back on.
As far as the stuck drum or rotor, I agree with the comments about using a torch. I got roasted for giving out this advice years ago, but the roasters were the ones who had never tried it, or didn't have access to a torch. First, you have to have an oxy-acetylene torch. Your little, bitty hand-held mapp torch isn't going to do it. You also need to have a rosebud torch tip (actually two tips with two people works better) and you have to hit it with heat hard and fast. The intent is to get the rotor or drum hot really quick before the heat can transfer to the inner flange. And, like posted, you hit the area with heat between the studs, not the studs themself. Quite often, the rotor of drum will give out a "pop" as it releases. If it doesn't, a few taps with the two pound (or larger) hammer will loosen it with a couple of taps.
