On my fourth cruise with HAL, I booked a HH (fully obscured, lowest outside catagory) class guarantee and was upgraded C (nicer window cabin). My daughter and I are booked on another guantantee cabin in November-- this time we booked the very lowest inside guarantee- We'll see what happens.
However, I would not book a guarantee when I have particular desires- for example, this last May when traveling with all three of the children Tours and Mr. Tours, I wanted to be sure our cabins were located closely together and therefore booked the actual cabins by number.

I am not sure what is meant by "scam" when referring to guarantee cabins. You are "guaranteed" a cabin at least in the class booked or better.