Matchmaking is the primary issue, although Gameplay Variance would also suffer from limiting the available ship options you could expect to see.
I don't have a figure to offer as to what would be needed to move matching to +1/-1. Our playerbase would need to be significantly larger to be able to accomodate something like that while still aiming for short queue times as a goal.
You mention other modes. Most other modes are single or dual tier specific, so that does already occur for those specific formats.