You may have me on the RNA gene count. ENCODE ruins all the best glib flippancies!
By cumulative time I meant the following: while each strain of bacteria has had the standard 3.5 Gya to evolve, there are many strains. Since every genome experiences this passage of time separately, this gives them a significant advantage in developing symbiotic and commensal relationships. (And, of course, they can test new mutations much more quickly.)
The rest, is, of course, reality; obviously the host can survive without its bacteria, and provides almost all of the colony's total functions. (And as a matter of fact, I'm studying such a minimal mouse gut flora at the moment.) I really just wanted to emphasize how significant microbes are from an ecological diversity standpoint, which was the context.