I'm not sure I'm confused about that (although it's possible that I'm confused about what I'm confused about), but let's see where this reasoning gets me:
Let's say that it's virtually certain that my opponent won't checkraise me (for the sake of argument, play along).
If I'm in position after my opponent has checked the river then the only two things I need to take into account is
a) will he fold a better hand, and/or
b) will he call with a worse hand
If a decent NL player calls a lot on the river, but doesn't bet a lot - a viable strategy - then all we need to make sure is that we're >50% of his calling range when we bet with a legitimate hand. The 99 hand above probably isn't, since there are a lot more K/T/flush/PP hands that will call than there are smaller PPs and A8 or AQ UI type hands that will. But JJ is probably okay to bet in that spot as long as we're sure he won't checkraise bluff us. Or?
Let me phrase it differently:
If he calls me with weak hands, I must value bet more. If he calls me only with good hands, I must bluff more. And here's what I think was my intuitive feeling from the first post:
If I only bet when I have either strong hands or bluffing with nothing, then I make it a little too easy for my opponent to snap off my bluffs since "strong hands" are rare and often fairly obvious on the boards. For instance, if I raise preflop and bet the flop, check the turn, on a board like the one above - a flush makes sense. But exchange that last card for the 5 of clubs, and what could I possibly have on this river that I'd bet now that I check the turn with? An observant opponent must decide that I'm bluffing a large portion of the time here. However, if I make the range of hands that I'm doing this with slightly wider, and include medium strength hands (at least some of the time), he will lose money the times that he thinks I'm bluffing, presuming that my hand is good enough to beat the average bluff catcher.
I don't know if it's profitable.