How will you use that data?
One of the beauties of technology today is how easy it is to collect data. Add a field to your database, set up a survey, collect the data. Couldn't be easier, right?
But there are several trade-offs involved (as there always are!). One of those is that odds are good that whatever data point you choose to collect will likely need to be maintained over time (i.e., it's not a static piece of data, like date of birth, but a dynamic one like area of practice).
And so when you decide to collect a new piece of data, you should ask yourself several questions, including how will you collect the data, who is responsible for collecting it, and who is responsible for keeping it updated.
But here's the most important question of all: what will you DO with the data once you've collected it? Are you collecting it to learn more about your audience and thus change how you interact with them? Or is it "just interesting" data? What action will you take?
My rule of thumb is simple: If the data you want to collect doesn't give you the opportunity to change how you do things, then it's probably "just interesting" data and won't be worth the effort to collect.
So be wary of collecting new data. Just because it's easy to do doesn't mean you should do it.