heat rises so I see no use in having a double wall bottom, a double wall top would make a lot more sense to me
Exactly. The double walled bottom, "feature", appears to me to be a "feature" that is looking for a problem. But cannot find that problem because of the fact that heat rises.
This is why hot air balloons work. I'm with you. In light of that fact, well then why not have an insulated top and sides too? Heat will be lost on a cold day from any openings or cracks between the lid and the rest of the grill, unless it turns out that when you close the lid on it, it's airtight, which we know it's not.
Insulating the bottom of a grill and not the sides and the top, makes as much sense to me as insulating the floor of the basement of a house and not the walls or the attic. And then selling the house to a buyer and trying to convince him that he won't have a heat loss problem.
Or worse yet, trying to convince him that he's somehow going to be way better off on his heating bills in the dead of winter, than the guy who's basement floor is not insulated.
That's laughable.
Heat will also be lost from the grease port, the smokestack, or vents, any openings, and from the skin of the grill itself.
Oh, and another vote for
Rec Tec. We love our 590. For the money, it's tough to beat.