Maybe you should ask this question instead: Does the phone company know your law practice as well as you do? Unless Ma Bell has been eavesdropping on your conversations, the answer is a definite “no”—which means it's up to you, and not the person who sold you the ad, to communicate what your firm does best.
Your advertisement defines your legal practice in the minds of consumers who still use the Yellow Pages to research businesses. Those consumers don’t give their trust easily. If you anticipate your ad bringing in even one client over the course of the year, you owe it to yourself to promote your legal practice in a way that will attract the clients you most want to serve.
The good news is that designing your own Yellow Pages ad isn't nearly as burdensome a task as you think it is. You probably already have a basic marketing “pitch” that you use on your website, so you don't have to write the copy from scratch. Word-processing programs are so intuitive that you can probably design your ad in half an hour (maybe an hour, if you haven't yet explored the capabilities of your Word software). If you have staff members, they would probably be delighted to take the file you give them and turn it into a bright, distinctive, attention-getting ad.
What will happen if you do, allow the Yellow Pages publisher to design your ad? Well, you may wind up with a lot of “white space” and a minimum of copy—often just the name of your firm, your slogan, and your phone number. The fact is that your ad rep has better things to do than worry about the layout of your display ad, and he/she won't be motivated to hit the ball out of the park, since she has probably already received her commission and has gone on to the next prospective customer.