IMO he was concerned about the flexibiblity of the tub bottom.
When a person steps into the tub, unless the tub bottom is well
supported (wood members, well spaced or lots of foam) the tub bottom
will flex downward.

It's a minor annoyance but can also lead to premature flexural failure
of the tub by cracking; cast iron doesn't have this problem.

I hadn't thought of that, but I think I'm okay. The tub, by design, is
supported by 4 4" squares evenly spaced. The floor itself is very
solid. Either way it's too late now. I already have the BB installed.
Thanks for the feedback just the same. Although I'd say my biggest
aggrivation with projects is over thinking things, it's good to hear
all the considerations.