I think that if they're not, they need to be in order for the community to remain healthy. My reasoning for this draws from two sites that have made it to the "Grown Up" phase of StackExchange that I've been involved in:
First, the full blown public sites:
- CrossValidated. This site has both some very basic questions (one of my most popular answers is on a pretty fundamental aspect of regression) and some extremely advanced questions by-and-for professionals, including some I've asked that have impacted my own research.
- Academia. I will admit I was worried about this site in the public beta phase, but it has graduated, and I think may be where the Health site might aspire to be. The vast majority of questions on the site are asked by students, are often somewhat personal in nature, but are dealt with by a supportive community, many of whom are senior or relatively senior academics. At the same time however, there is definitely room to for those senior people to ask questions and get advice from their peers.
I think the key to both of those sites is that the incentive to post there by professionals is to foster the kind of place where they can also ask question - I have a close to 1:50 Questions to Answers ratio on Academia, but those questions that I did ask were very important to me - and got answered, and answered well. I ask a lot more questions on CrossValidated, but my ratio there is still about 1:6. But for both places, there's a strong incentive to participate in order to foster an environment that, selfishly, will someday benefit me.
If you remove that, then answering questions on this site becomes purely an exercise in public service for the experts posting here, and a public service that they will neither be paid for, particularly recognized for, or professionally rewarded for (in contrast to perhaps StackOverflow). While that may be sustainable for a few dedicated few, it runs the risk of having most experts on the site come to view it as "Something to browse when I'm on SE for something else and have a few minutes", which is not I think a particularly healthy place to be for a site.