If I need a bit more suspension, I can just put 5 psi less in the tires, it seems like the most logical place to have, well, flex.

The power losses may be negligible, but you just couldn't argue in favour of "more frame flex".In the modern days of CF goodness you can have a nice, stiff bike which is also light and aero (and good looking to boot), and which feels really solid when you're descending at speed.