Bladestorms are more of a hit and run kind of troop. After you get them to level 3 and upgrade their Stamina they become really useful.
They have extremely good attack but their main strength is the whirlwind ability. The best use is to change to Wind formation, cloak, run into the mass of enemies, perform whirlwind twice, and escape. With this strategy they can easily take out 2 battalions for each blade-storm battalion with minimal damage.