Also, if you promote a BOff to Lt, they can use all of their ground skills. Just because they don't have a 2-skill console in space doesn't mean they can't use the additional ground skills planetside.
And those additional ground skills can be very helpful in combat.
ETA: Another possibility to consider is to wait until you get more requests by other Star Fleet officers to transfer to your ship (the little "mission" that gives you a new BO as a reward every few levels). Some of those new BOs have great traits that help out ground combat, so you might want to keep slots and training points open for them.