Underpitching may or may not be a way of developing flavors. There are conflicting opinions and no hard evidence that I'm aware of.One of the talks I went to at NHC said that basically there is strain dependency in this, so one strain may develop more yeast character when underpitched, and another when overpitched. Sounds like testing and personal experience are the way to go.
"Standard" pitching rates seem like such a WAG to me that there's no way they're the "best" pitching rate for every yeast and every style. Wyeast says <1.060 and you need 6m cells/ml, while over 1.061 you need 12m/ml. If you go by the (0.75m)x(degrees plato)x(ml), then a 1.061 beer would need 11.25m/ml. I doubt Wyeast would publish that pitching rate for their products if it didn't give acceptable results most of the time for most brewers.
A lot of people talk about "overpitching" and "underpitching" and some people claim success and others failure with either. I suspect there are a dozen reasons for the differing outcomes, like the brewer's fermentation controls, yeast health, oxygenation levels, brew process, etc. I'd say any amount of yeast you pitch to get the flavors you want is the "correct" amount to pitch, and any rule-of-thumb on pitching rates is just that, a rule-of-thumb.