Online Dominion is many different things to many different players. Some play (almost) exclusively against humans they never met in real life. Others play (almost) exclusively against humans that sit 3 feet away from them. Still others just play game after game against a bot.
Hiding the bots will just anger the latter player base for no real gain. If somebody wants to open a Dominion Academy that teaches better Dominion play, I'll be the first to sign up, because I really need it. But most beginning players will not get better if they are being beaten by "Lord of the Engine" instead of "Lord of Big Money".
Just as many other suggestions, better (and varied) AI would be nice, but takes time to do.