What's up with skipping the convention?

Prominent Democrats like Sen. Claire McCaskill (MO) and Sen. Joe Manchin (WV) are making a point of not attending the Democratic Convention this year.  I just don't understand the motivation for such a maneuver.  It's not like voters aren't going to see the "Democrat" next to their name right on the ballot.  Voters know McCaskill and Manchin are going to vote for Harry Reid for Senate Majority Leader.  So  their decision to skip the DNC therefore has no real benefit.  But it inflicts a real cost on the party they supposedly represent.  These Senators represent rather conservative states, and they fear being tied to Obama and the national party if they attend the convention.  Refusing to attend the convention gives a signal that the party is somehow so extreme that they can't even soil themselves by talking with the rest of their party for a few days.  This puts pressure on other candidates, as Democrats who do go the convention are now labelled as more "left" than even other Democrats.  Once it becomes a thing to not attend, it slaps an unfortunate label on those who do.

So I would love to ask the Senators just what the Democratic Party has done that merits such a response.  How are they so disenchanted with their party that they can't even attend the convention and, I don't now, try to save the party from itself?  If you're doing to have a D next to your name, along with a promise to vote for a Democrat for your leadership, then suck it up and go to the convention.