Why use Ephesians rather than Romans or Galatians when teaching about being in Christ?
Ephesians is used because it presents a deeper, higher level of Christian experience. Paul brings out truths in Ephesians that he does not bring out in Romans, Galatians, or Corinthians. It is described as higher ground and deeper water for understanding what it means to be in Christ.
Les explains that Ephesians is a deeper level of Christian experience, or a higher one, depending on how you look at it. Paul will bring things out in Ephesians that he doesn't even bring out in Romans and Galatians and the Corinthians, because this is higher ground and deeper water. Thus, Ephesians is used for an explanation of what it means to be in Christ, going beyond what those other letters cover.