If you want to increase the number of useful discussions, increase the number of non-useful discussions.
Look at this image:
From a conveying useful information perspective, the most popular discussion has 0 value. However, from the perspective of encouraging people to bond and become emotionally invested in the community, it's very useful.
When members become emotionally invested in the community, their level of activity goes up. When members participate in bonding/status-jockeying discussions, their level of activity goes up. When members get to know each other beyond the topic, their level of activity increases.
This doesn't mean posting questions in your community of practice asking what your members ate for breakfast, it just means shifting the balance of discussions. Ask members more for their opinions on a topic as much as you ask for their knowledge/information.
This works in almost all types of communities. Business Fights Poverty (a client) does well to feature discussions that convey information, those that ask for opinions (bonding discussions), and those that encourage elements of status-jockeying.
It's not difficult to find discussions that ask members what they think about relevant issues in the sector or ask experienced-based/hypothetical questions about how they tackled/would tackle specific problems.
If you do this right, members will visit and participate in the community more frequently. The number of discussions that convey information will be a much smaller % of the overall number of discussions, but there will be far more of them.