I'm persuaded of the value of this approach for several reasons.
Among activists who very much want impeachment, one can hear a long
list of fears and concerns about how things might go wrong, how
impeachment could help Republicans who come around and back it, how
impeachment could take energy away from elections, etc. But by far the
most common of the nonsensical fears one hears is this one: "Impeaching
Bush would give us Cheney, who is worse."
By proposing to impeach Cheney first, we eliminate this fear.