Attacking your opponent means you dictate what they have to speak about. It gets them off their own message. It's American Politics 101. Kudos to Obama for trying to avoid that dirty pool up till now, but he can't play that way forever. There are plenty of shady land deals in Hillary's past, as well as quid pro quo Kazakh oil deals, senior advisors who awarded tens of millions of dollars in compensation to sub-prime mortgage company chairmen, criminal fund raisers, etc..., etc..., etc...
If Obama doesn't want to start playing those cards now, he won't be around in the fall when the GOP interest groups start playing them.