Under normal circumastances, your logic of keeping HIllary in the race until Obama secures the required number of delegates would make sense. However, these are not normal circumstances! Hillary is unfairly attacking Barack Obama at every opportunity, her low blow attacks are not only vicious, but she attempts to give the perception that he is the enemy rather than a Democratic colleague! The longer she is allowed to continue to attack Obama in this way, the more irreparable damage will have occured. I can't see any benefit in keeping her in the race longer because her attacks are not only affecting the overal morale of the race, but it is also affecting her own supporters who believe what she claims in her attacks against Obama and therefore keep forming more negative yet unfair opinions of Obama. How is that any good?
I say give her an ultimatum. If she wants to stay in the race and loose gracefully, fine, but she must stop attacking Obama as if he were the enemy. And if she has a need to attack someone, she can then attack the Republicans until the cows come home, but she needs to back off the unfair Rove style attacks against Obama. Then and only then does your suggestion of letting her stay on make sense.
Hillary Clinton has become toxic to our democracy, the Democratic party and her own supporters.