By trying to impeach Musharraf instead of restoring the Judges the political parties are accepting him as a legal President. Should the impeachment drive fail then Musharraf will have effectively been confirmed as the President by the National Assembly. The Judges issues then becomes a non-issue even if the Judges are restored and Musharraf can continue for 5 years.
You are absolutely right that if Musharaf gets a vote of confidence from the Parliament, then the judges issue will go away too. Problem is that its not gonna happen. I doubt impeachment will ever get to a vote. Writing will be on the wall in the days leading up to the impeachment vote. Expect a deal! :-)