Are Micro A USB plugs actually ever used?

Yes. Pretty much ignored in favor of the micro-B. There is no "Micro-A receptacle" since the micro-AB covers both. There were many more orders for micro-B cables than there were receptacles, and the industry quickly scrapped the micro-A.

The definitive answer is in the USB OTG specification, the current revision is 1.0

On a quick read, Section 3.13 states

The OTG supplement defines the following additional connectors: Mini-A plug and receptacle, and Mini-AB receptacle.

and

Any device with a Mini-AB receptacle shall meet all of the requirements and provide all the functionality of a dual-role device.

You can check the details, but that implies that presence of the AB does require all the functionality, but there are connectors defined which could be used on an OTG device which doesn't meet all the functionality.

If you find Wikipedia is incorrect after you've read the document, please share the knowledge and update it with specific references as well as confirming here.

It is rather uncommon.

You would find it in a device that is so small that a standard A connector wouldn't fit, but where the device would still be host only. I can't think of any good examples quickly, which should give you an idea why they are uncommon.