Each one has a triggered or activated ability that gives them a four-color identity, and each of them is Ink-Treader
For a three-powered dork that's good since Looting for three will draw and then discard what you don't need or cannot cast
There really are that many
In case you've heard the term but don't know the cards, here's a rundown of the original Power Nine: Honorable Mention: Library of Alexandria, arguably Magic's most powerful land, rounds out most people's list of the Power Ten
The ability to add an artifact, for free, to the table is just really strong
This gets better the more players you have in your games, and so is naturally at its worst in Cube, where cards at this mana cost get cheated into play and do a lot more to win the game than come with two [card]Phyrexian Arena[/card]s