This debate is framed in a vacuum as far as I'm concerned. I can't say I've ever been to a brewpub that serves BMC. Seems highly unusual. I guess others have, based on comments.
My local brewpub had a Miller Lite tap for years. The brewer finally convinced the owners to drop that for a guest tap a few years ago. He was still stuck with having two fruit beers on all the time, though, as the owners insisted. A lot of this is driven by the local demographics. Their American wheat is their best seller by a long way, I'm told.
I dunno, I'm kind of enjoying my armchair publican role here.
For that matter, have you ever seen other micros on tap (other than guest brews, collborative brews, or the like) at a brewpub? I haven't. And if you ran a brewpub, would you have a tap for Sam Adams, Sierra Nevada, or Stone? I wouldn't.
The local has had an interesting range of guest beers, including a firkin of Surly Darkness one splendid night. My goodness, that's a great beer.