Obama supporters demanding Hillary withdraw from the contest immediately need to relax. At this point it is all but certain Obama will win the nomination. But that does not mean that the Democrats should end the Obama-Clinton contest. On the contrary, the only thing that would achieve is to allow the GOP to start their slime-fest.

In particular we can do without the attacks on Clinton for daring to actually continue to campaign. The folk behind them need to consider the fact that the only way that an Obama-Clinton ticket makes sense is if the party is so divided that this becomes necessary for party unity. Clinton is not going to want the veep slot, Senate leader or SCOTUS member make much more sense for her. The only reason to take veep would be if she was likely to be viable as a successor. I don't think she wants to become President in 8 years time.

As a purely practical matter, Clinton cannot withdraw from the contest on a win, not unless Obama has actually crossed the finish line - which is not going to happen.

There is also the issue of party building and turnout. This is particularly important in Puerto Rico where a closely fought primary campaign may well have implications for the next statehood referendum.
 
Finally there is the question of finances. In particular the large sum that Clinton has already collected for the general election. The funds cannot be transfered directly but donors can be asked to re-allocate the funds.

There will be a presumptive nominee by June 15th and it will almost certainly be Obama. Pretty much all that could stop that would be a Spitser type situation or Barack falling under a bus. But there may well be advantage to maintaining some degree of uncertainty right up to the convention.

The Republicans were only able to launch their smearboat attack on Kerry in a critical window between the end of the primaries and the start of the fall campaign. They could not have pulled off the same attack later. If they had attempted it before the convention it would have backfired. Any pre-convention smear would have been easily countered by pointing to the fact he was nominated anyway.