Ships is the only actual combat thing it will help, if they're a pilot. Otherwise all it does is raise your GP, which helps a small amount in Territory Battles. Otherwise, nope, nothing.
Personally, I go for it if I've got plenty of the type of mats, don't bother if I don't. Which means that for a long time I stopped leveling leader abilities I didn't use at 3 (except for pilots), and only recently have I wound up with such a surplus of mk 3 mats that I've taken them to 7.
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So...depends if you use them for ship battles or have tons of ability mats.
Personally, I go for it if I've got plenty of the type of mats, don't bother if I don't. Which means that for a long time I stopped leveling leader abilities I didn't use at 3 (except for pilots), and only recently have I wound up with such a surplus of mk 3 mats that I've taken them to 7.