Come to think of it, Tenryuu's right. They won't actually be using their legs while they're in battle, they'll be in their "ship form".
They already said that their ship form depends on their land form, so strengthening their legs should presumably up their nautical speed and fuel economy.
Flashback fin.But, just what does he expect to get out of us, strengthening our legs like that?War's about tactics and strategy - there's lots of other places to strengthen, yeah?