Because your max overclock will be the same, no matter what the multiplier is set to. The only gain you'll make is that your memory will run a bit faster because of the higher FSB needed, providing you use the same memory divider. And the gain is pretty small tbh.
And you'll likely need to set higher chipset voltages to get to 3.4ghz using a lower multiplier.
Tho that's not to say that you shouldn't run it on a lower divider. Plenty of us do.