RBs are a dime a dozen (unless they are elite). The Patriots were thin at RB but could have easily picked 4th or 5th rounders and a few low end free agents.
I disagree with this. If you consider the top half of starting running backs in the league, a huge percentage have been drafted in rounds 1-3. It is unusual to have quality backs taken outside of the top three rounds.
From the top 20 rushers as of today:
Backs taken in rounds 1-3
MJD, LeSean McCoy, Frank Gore, Ray Rice. Matt Forte, Marshawn Lynch, Ryan Matthews, Beannie Wells, Willis McGahee, DeMarco Murray, Steven Jackson, Cedric Benson, Chris Johnson, Adrian Peterson, Shonn Green, Ben Tate and Reggie Bush. (McFadden and Jamal Charles have been injured this year, and Mendenhall has struggled this year.
Backs taken after round 3 or undrafted:
Michael Turner, Arian Foster (keep in mind that Tate is averaging 1.6 more yards per carry than Foster), Fred Jackson
While there certainly are exceptions to the rule- and Helu looks like one- it is rare to get a quality back after round 3.
RB is a position that usually requires top notch athleticism. There is not a huge learning curve in running the ball. THe curve is only in blitz pick-up. Most guys show that they can run early in their career. Far, far more often than not, if you want a useful NFL back, you have to be prepared to draft him in rounds 1-3. NE may have drafted the wrong guys, but it was no mistake to draft guys in those slots.