I'm not saying a top RB isn't helpful, only that the top resources are better spent elsewhere. Take a look at that list of top rushing teams and how they acquired their "lead" back:
1. San Francisco 49ers (Frank Gore, 3rd round pick)
2. Kansas City Chiefs (Jamaal Charles, 3rd round pick)
3. New England Patriots (Stevan Ridley, 3rd round pick, split time with LeGarrette Blount, acquired in a trade for Jeff Demps, a former 7th rounder)
4. New Orleans Saints (Pierre Thomas, undrafted)
5. Seattle Seahawks (Marshawn Lynch, acquired in a trade for two low round picks)
6. San Diego Chargers (Ryan Matthews, 1st round)
7. Green Bay Packers (Eddie Lacy, 2nd round)
8. Denver Broncos (Knowshon Moreno, 1st round)
9. Cincinnati Bengals (Ellis was a free agent, Gio Bernard a 2nd rounder)
10. Carolina Panthers (DeAngelo Williams, 1st round)
And it's not like the first rounders are the best players on this list.
Not that I particularly like the idea of spending first rounders on offensive linemen, either. But if you look at those top teams it's full of high round offensive linemen. Kansas City, New England, San Francisco, Seattle and Cincy all have multiple first rounders on their offensive lines (I'm not going to look them all up
)