There are ways to play dragoons very efficiently, but it requires more effort than other classes. If you learn to form your firing line as fast as humanly possible in favorable terrain, you can do a lot of damage against infantry lines on close to medium range. The distraction value is also huge - any infantry line under fire from mounted dragoons will immediately redirect all their shooting there, regardless of distance, allowing friendly infantry to win. As for melee, the heavier dragoon classes are comparable to heavy cav, and the lighter are comparable to hussars, in both cases slightly worse but far from worthless. I find that the best place for cav in NW is supporting infantry in melee engagements, where there's an abundance of targets not in the mindset of defending against cavalry. Dragoons have the option of taking a very close-range volley before engaging in these situations, which has on several occasions netted us 4-5 kills.