It's possible that the "you can't shoot weddings on an APS-C" argument may be a simplified version of "none of the APS-C cameras currently available have the features a wedding photographers need". An example of such a feature is redundant memory cards. Then as more professional APS-C cameras were introduced the argument became invalid.
And how do you define "better sensor"? I can always find a case where a "crap sensor" beats a "good sensor". As an example, if you're shooting a closeup of a helicopter in flight, the sensor in a $150 point-and-shoot will outperform a $5000+ Canon 1Dx, just because of the shutter design. And the shutter is an integral part of the sensor for CCD chips.
And how do you define "better sensor"? I can always find a case where a "crap sensor" beats a "good sensor". As an example, if you're shooting a closeup of a helicopter in flight, the sensor in a $150 point-and-shoot will outperform a $5000+ Canon 1Dx, just because of the shutter design. And the shutter is an integral part of the sensor for CCD chips.