My theory is that they're just placeholding for now. They don't really care about investing too much in it or their present users. Circle's occasionally spouted off about tokenising everything which requires an exchange.
However they may find that if this plan ever comes to fruition no one's going to want to go anywhere near Poloniex.
That's the thing -- wasn't the purpose of buying them to leverage the brand? Letting the brand go to shit seems like a good way to shoot themselves in the foot. Placeholding made sense over a year ago but my guess is they've run into unexpected regulatory snags that have piled on the delays. I can't imagine things are going as Circle planned.