No, you are correct.
Personally, I assign all un-assigned officers manually after auto-assign.
This does two things:
One, as you said, it lets them improve. Officers without any skill often get the skill relevant for the job.
Two, it keeps them from being fired for not being employed and when my fleet building program kicks in (I normally play conventional start), I usually need every R2+ officer I can get my hands on.
Note: Ground Unit officers don´t imporve skills "on-the-job", only in combat.