clean slate at the end of each iteration

Unfinished stories mean:

• stories that won't be part of this iteration's deployment, and may have to wait another full iteration for full feedback
• no slack time and therefore wear and tear on the team
• no clear moment in time when all work-in-progress was done-done, so it may be meaningless to tag a release. That is, partially implemented features infect the iteration's release since there is no moment where all committed work has been fully acceptance tested. Yet another way to say that is the code base is left with inconsistent horizontal/technical components added along the way to implementing a story/vertical slice.
  
• no clean slate at the beginning of an iteration, so it can be harder to "turn on a dime" with a new slate of stories--the client will want to finish the ones already partially paid for
• "yesterday's weather" is much more muddy--how much work is left on these unfinished cards?
  
• loss of credibility/accountability
• treadmill effect (without the punctuation of a clean finish, where's the energy of a clean finish and fresh start?)