Nice deconstruction of the old analogy. Come to think of it, I only use my belt as a fashion accessory ("if you have belt loops, wear a belt"), sometimes I skip it. Getting your pants fitted right and maintaining your shape beats a belt and suspenders.To avoid losing my pants, I make sure I have a proper fit and use a belt. There really is no need for suspenders, too.
The same applies to diving. A broken-in, tightened band with a correctly threaded buckle will not slip. For novice divers who can't reliably ensure that, the cheap "safety strap" can even be better, because it's a different mechanism, while both bands can fail from one problem - never wetted, improperly threaded buckles, etc.
Actually, this thread has lead me to a minor personal decision: the next time I travel for single tank diving, I'm going to leave my second tank band at home. It's not a lot of weight, not even with my SS buckles, but it's weight I don't need.