I think there's a lot of value to your suggestion. Not just for designers, but in all industries, focus is vital and at a least a rudimentary understanding of your client's business can get you off to a running start.

There are some problems with the idea to consider though, and I point these out as someone who briefly tried focusing most of my energies on one industry.

*Client references: if they think you did a good job for them, why would they refer you to someone they consider thier competition? If they are dissatisfied with you for any reason though, you better believe they'll tell everyone.

*Expectations: The fact I had a good general understanding of the industry I was focusing on and access to someone with a pretty encyclopedic knowlege of it, worked against me when I did ask questions. Some things are specific to individuals and individual companies - and they don't always realize it themselves. If you know a lot, they seem to expect you to know everything, including whether thier wife's cell phone number should be the primary contact channel or not, without being told.

I'm not saying the idea isn't a good one, I'm just saying there are some things to consider.

These problems, although they were problems in my experience, weren't the reason I abandoned my first attempt at focusing on an industry. It was the people in that industry. Partly because of the demands that industry puts on the people in it, and partly because of the type of people the industry attracts, I just didn't want to spend all of my time dealing with it. I still think it's fertile ground, and I may go back... but let's just say I'll never consider a full-time career in that industry.