It seems natural that good UUs make for bad upgrades. I don't care for Gallic Sword to Medieval Infantry, for one thing. I keep all of my Gallic Swords even when MDI are available, and just train knigts. Gaining one point of offensive power is not worth losing the extra movement, even at zero cost, and GS can take on even Cavalry, at least on the offensive, and with luck and good management can be used against Muskets. As for Dromons, they're one of the best UUs in the game, and I keep them until approximately Ironclads myself.