Yeah, its a bit of a shakedown, but they not only can avoid any harm coming to your bag, they can also make things work out for you really well. They actually share their tip money with the baggage handlers, so the money is getting to the right place.
Once I was flying from Phoenix to Atlanta with my golf bag. I got to the airport later than I should have, and there was an amazingly long line to check bags. I waived over the sky cap, and all I had was a $20....I told him I was in a rush and he said not to worry about a thing. My bags were checked, I was on the plane in plenty of time, AND my bags were the very first ones down the conveyor belt in Atlanta. Well worth the money, IMHO.
As Steve Martin said in 'My Blue Heaven', "I believe in tipping everybody - No, I believe in OVER tipping everybody."