Why would one shoot strictly for equivalence?
The original comment was that a 5D2 lets you stop down more before diffraction impacts sharpness. This is false. When shooting equivalent landscapes (FoV and DoF) diffraction does not impact any format more than any other. You cannot get "more DoF with less diffraction" with FF. This is true across the spectrum, from 4/3 to large format.
I'd rather shoot a 5DII at ISO 3200 in a dark auditorium rather than a 7D at ISO 3200.
For me it would depend on other factors. Again I'll point out that if you need more DoF then you can get with a wide open prime on FF, then you've lost the sensor ISO advantage. f/1.4 is not a sweet spot for FF, it's hardly usable. And I have more trouble nailing focus with a fast prime on the 5D2 then on the 7D, aggravated by having the lens wide open.
Now if we're talking f/2.8 zooms where f/2.8 offers sufficient DoF, then yes, I would pick up the 5D2 for the dark auditorium. The f/2.8 zooms make complete sense on FF for, say, wedding and PJ work.
The same lenses will also result in higher resolution (lp per image height) on a larger sensor.
A false meme caused by a general misunderstanding of lens testing. The same lenses will result in higher detail contrast (sharpness). If these were the film days, that difference would be a big deal. In the world of PS it's inconsequential between these two particular sensors, except at higher ISOs where sharpening increases the noise differences.