My short answer is always, "YES!"
The somewhat longer answer is not only do I think conferences are important, I consider them a crucial strategy for advancing my career. Here is why:
1. 99% of the time, our communication with editors and potential agents happens over the transom via snail mail and email. At a conference, you see, hear, smell ? these folks, and even if it happens at a distance, you get a sense about them that is impossible to find in resource books or internet bios. This sense can be infused in your cover letters and queries. And you can weed out those publishing houses or agencies that aren't a good fit for you or your work.
2. You bypass the slush pile. Faculty often offer windows during which your work will be read if you attended the conference. Pay attention to their instructions for submitting. They may ask for a particular phrase referencing the conference as a subject line.
3. Your work can be evaluated by people in a position to buy it! Almost every conference offers first pages and/or critiques by agents and/or editors. Take these opportunites seriously. Register early and send your best work with the knowledge that it will be evaluated. Even if the piece critiqued at that conference is not requested (which is most often the case), I've have several critiquers offer to read more work in the future. Additional fees will apply but I've found these interactions to be invaluable.
4. You can experience the reaction of industry insiders to your work in real time through "first pages." Many conferences offer this session during which a panel of experts react to a first page of a novel or picture book. It is incredibly scary, but exhilarating to hear roundtable comments to your work! Just get over yourself and go for it. It's going to be a learning experience no matter what.
I say, "YES!" Add conferences to your expense column. You'll be glad you did.