There are a few different things to consider here: as others have said, grounding is not the same thing as bonding. Grounding implies a connection back to earth, and if the legs are aluminum, you already have some amount of "grounding" just by them being on the ground. If the ground is wet and the aluminum is unpainted and in direct contact with the earth, you already have an excellent ground, whether you want it or not.
I disagree with Cailen about being sure that the stage would be a less-preferable path than the actual ground wire - there's potentially a lot of metal with a lot of ground contact, not to mention that there's no way to ensure that every piece of equipment is always properly grounded; - an SO cord could become pinched between stage sections causing a fault during the actual performance, even if the cord was in perfect condition a moment ago.
<snip - generator discussion>
For life safety, you need to think about the potential for fault currents. If a metallic object has the potential to be energized - something very possible with lots of power distribution and instruments of unknown condition right on your conductive deck, then it should be bonded to the electrical service, such that GFCIs and OCPDs will function. This likely also means the metallic object is grounded, but not necessarily (see below).
If you are using shore power, receptacles should be GFCI, and the decks should be bonded together and to ground.
Thanks, TJ, for some very salient points. Some of it is a bit confusing, and some may, according to my reading here, may or may not be in conflict. I'd like to explore the answer a bit, if you please.
One thing that is confusing my direct situation is the generator discussion. If you please, I am very interested in that, too, but I'd like to explore that a bit in a different post.
I'd like to explore the shore power aspect of your answer in this post.
First, I have to assume the staging in question (aluminum deck and legs) is poorly grounded as it sits. I have never worked with the staging on anything but a slab of concrete. (wet and dry). The legs have a vinyl cap to protect a surface from scratching, acting as an insulator. The square legs fit loosely into sockets. Aluminum oxidizes quickly left open to the air, and creates a high resistance surface on the aluminum further degrading the natural grounding.
Second point is grounding vs. bonding. As I understand it, and discussing only shore power, the safety ground in the GFCI eventually is connected to a ground rod somewhere (close to the power panel, I would assume). Neutral would be bonded to ground in only the first panel (and only bonded once - not every panel/sub-panel) after the power leaves the pole. (lay explanation) So, in a shore power situation, bonding and grounding are the same, correct? (Understanding gennys are a different kettle of fish)
Third point: "... deck, then it should be bonded
to the electrical service, such that GFCIs and OCPDs will function" I'm assuming this is a comment for generators, as your last statement "If you are using shore power, receptacles should be GFCI, and the decks bonded together and to ground"
Fourth point: I fully agree - any cord on this type of stage can (and has) been caught between sections, though we've been lucky to have caught them before the cord insulation could be compromised.
Fifth point: I think I've read here, that just because metal is in contact with the ground (wet or dry), that constitutes a good ground. I'm out on a limb here, but I'm thinking that just being in contact is inadequate given the length and composition of the grounding rod.
And thanks for helping me gain a better understanding.