Taking a brief look at
answers.ros.org it appears to be an Askbot based Q&A site created by ros.org to support their community.
As a more vertical site, you may get more ROS specific answers there, but equally ROS is perfectly on topic here. Just because you could ask a ROS question there shouldn't stop you from asking it here.
Similarly though, we shouldn't be trying to steal traffic from
Personally I don't think Askbot (or OSQA) based Q&A sites work as well as a properly defined, committed and public beta'd Stack exchange site, but then they are still playing catch-up, both technology and community maturity wise.