I was always under the impression that the armour was some sort of special material, as well, lightweight to wear, but still protective. Considering how little it actually protected its wearers, the wearing of it could be completely ceremonial. Of course, it could also be that the species that the Union had to passify before we see them for the first time on Trek had inferior weapons, and the material the armour was made of actually did serve as adequate protection, but then, we see many Cardassian soldiers taken out by Cardassian weapons, sooo....yeah.
Perhaps it was originally designed to protect against hand-to-hand, ground combat situations, and the design simply never really evolved to adequately protect against energy weapons. It may serve to dissipate a blast somewhat, but it's still no match for higher energy charges.