Bumping is fun, and if you can keep it in a controlled pattern, I don't see anything unsafe about it. As for it being a waste of money...well...it may be, but that's not your business, that the business of the guy buying the ammo. Don't tell me what kind of car to drive, don't tell me what kind of gun to buy, and don't tell me how much ammo I can burn up!
I can bump my AR and my AK-74 from the shoulder. I've yet to try it from the shoulder with my 7.62 AKs. Some even say bump firing is a practical and effective (for Zombie attacks and the like) shooting technique, if properly mastered. I doubt it, I think you're better off mastering your ability to quickly regain your target picture and fire another shot. But Bumping is fun and you'd be surprised as to how good you can do it if you practice.