Dweezil Zappa

I've been thinking about starting a marketing agency for a while now, along with other things, and I was just wondering if it would be a good idea to do marketing in the legal niche. I always see people going to restaurants, gyms, and medical offices and doing marketing there, but I've never seen/heard of an agency doing marketing for a law firm.
Do you think it would be a good idea to, say, go to an insurance lawyer or one of the other areas of law and do paid advertising for them?