Basically this. We're asking an incredible amount of our centre backs really, because they're the only defensive-minded players in the team. Both wing backs are now genuine out-and-out wingers, and neither Palmer or Bostock could be described as natural holding or defensive-minded midfielders.
It means that when a team or player gets on top of us for a spell we are extremely vulnerable, but it's happening rarely enough that the good massively outweighs the bad.