How often? Every time I need to power cycle it. And the odd thing is..you might think its a small thing, but most designers pay attention to the small things. Ever notice very few routers have power buttons? I think like..2 on the market? They were concerned people would accidently press it. Notice how Motorola added a standby button from the SB5100 to the 5101, then patched it in the 5120 to turn it off, and then complely removed the button in the 5101U. Not to mention the power adapter is cheap compared to every other adapter I have bought from D-link. Shorter length, less insulation, etc. I think they just got a deal from some no name vendor in Tianjin.
But like you said, its not a big deal. Just seems strange to skimp on your flagship model.