the plat is not useless, but greedy wb&turbine shifted your buying power from plat to shards, premium currency, injected in the system by players usually with real money. so obviously you'll try to sell for shards, cause they have more buying power. coupled with the 30% fee, the system is designed to slowly make premium currency vanish and setting the need to be re-injected. this is true both ways, both premium currencies, shards and ddo points....if you get shards via ddo points farm, the system is designed for you to gain zero, except buying power, but people like the idea of gaining shards because they cost. you think to gain something but you gain zero, except barter power and this is not a small thing anyway, it's very welcome, that money was cashed-in already, using it will burn the shards someone else did buy.
it's too late now for a game economy, instead of fixed it was replaced by shards, it was destroyed on purpose instead of being fixed, we can forget about it.