Just smoke - I totally agree, and my feeling is also based on what others have said about different liquids in the pan not flavoring in the meat, in their experiments.
However, I was planning to do water only - as a heat sink, yes! But also, just to see if the slightly higher % of humidity in the chamber, might get my ribs just a LITTLE bit more tender.
Reason being, like I said in my earlier reply - I don't really want to foil my ribs. However, sometimes I wonder if just the water as a humidifier as WELL as a heat sink factor, can make the tenderness quality more consistent for my rib smokes.
Logically speaking, even if the water pan steam doesn't really penetrate the ribs for any more tenderness, just having a heat sink added will keep temps more steady, always resulting in a better product. Therefore, you are correct that I could just use sand, or blocks. However, I'll use plain old water pans for starters and see how it works.