I have seen related discussion on discord. Here is my idea of mechanics that encourages using units not too often and not too rare.
Units gain new dynamic stat - Thirst (meaning blood thirst or battle thirst).
It changes in the range from 0 to 9 and is shown in hands tables such as "select crew" or "hands" on the far right.
It ticks up by 1 per week. If unit goes to combat, it ticks down by 1. Also, if unit has above 5 Thirst in battle and does some exp-granting activity, it ticks down by one and this exp gain is tripled. So, if Hand goes to combat once per month and actually is doing something there, she will not lose stats (and will have nice exp bonuses)
If Thirst is already 9 and has to tick up, it drains stats, because soldier is out of practice. Something like -10 to 3 different stats at random.
If Thirst is 0 and unit goes to combat, it stays 0, but unit has significant temporary stat penalties during this combat - such as -20 to each stat.