I own a shop that works exclusively on Toyota, Lexus, and Scion. It has been great, business has grown steadily for 2 years. Most of my customers come to me because another shop that 'works on everything' usually can't fix whatever problem they are having or have screwed up their car and now don't know who to trust. So they come to the specialist. Once we fix what the other shop screwed up or couldn't diagnose, we've usually earned their business and they don't take their Toyota/Lexus anywhere else. Also, I am in a 100K+ population city with no Lexus dealer, so the Lexus owners don't have much of a choice besides me or a 2 hour drive.
All that being said, I had my website design people add Honda & Acura today. Why? The volume just doesn't seem to be there for Toyota and Lexus alone. Like I said before, the business is still growing as more people hear about us, but if I want to reach the sales goals I have in mind, one line of cars won't cut it. Adding Honda/Acura will still allow me maintain the 'specialist' name/niche, but adds more volume. It also lessens the blow on additional tools I'll need to add. I also think it makes it a little easier on the staff, since they see the same cars/problems every week... My female service advisor with no service history can almost diagnose cars before she hands me the ticket on them since we see repeat problems over and over again.
I think it's really in how you market your shop. I made sure to get every certification possible before I left the dealership so I could hang it on the wall in the waiting area. I also use the factory scan tools, allowing me to do EVERYTHING to the car (keys, reflash, etc). Having the 'specialist' name, the 6 certificates with my name hanging on the wall, plus the certs of other staff, and advertising 'factory scan tool' allows me to charge a higher labor rate than most other independents and few people price shop me (They already expect to pay a premium at the 'specialist')
Hope this helps somewhat. In short, I think it makes sense to specialize in something, just don't narrow your market too much.
Also, the back of the shop is a great hiding place to break the rules. We've done an oil change on a 2015 supercharged Jag and have a customer with a mazda who spends over $500 every visit... Yet we're still the Toyota and Lexus Specialists