1. Better traits:
Efficient for Space, situational for Ground. I have two sets of BOffs, one for ground and one for space.
2. Exclusive skills:
There are abilities that cannot be trained, and must be obtained from Uncommon or higher quality BOffs, such as Aux to Structural Integrity 3. Note that you can always use a candidate to train your already commissioned BOffs, though.
What ranks those skills are at is irrelevant - you'll accrue more than enough Expertise to max BOffs in one go, while spending it on DOff assignments and Fleet projects.
"Don't let them promote you. Don't let them transfer you. Don't let them do anything that takes you off the bridge of that ship, because while you're there... you can make a difference." - James T. Kirk