"Might" being the operative word.
It isn't quite an appropriate comparison since luthiers aren't as common as auto mechanics, but given a choice between a guy who openly voices his dislike of a brand, and a shop across the street that specializes in that brand, and reasonably comparable pricing and turnaround, who would be most likely to get your business?
Let's toss in another complication:
The guy who hates Fords is quoting $200.00 less for an ABS repair than the Ford specialist.
When you ask the guy who specializes why that is, he can explain what it is about Fords that justify his price.
It's how I wound up with my RX-7 and Ford mechanics for many happy years (and they were actually cheaper than the dealerships, ex-dealer employees themselves).
I did get what you meant about folks who have the integrity to do good work even if they wouldn't own the brand themselves, but what I should have said is "Why take the chance if you've got an alternative?"