I didn't notice if this was mentioned or not, but draft position also matters. Usually there are "tiers" of players. If you're drafting 1st-4/5th you're probably taking a super stud RB over anything, in the next few picks, one of the big 3 QB's is probably going to be gone and possibly Randy Moss or TO.
So say the top 8 picks are 1-5 (Stud RBs), 6 Brady, 7 Moss, 8 best RB available ...
After that the next 5 or 6 RB's are probably fairly close in value and if you're drafting, say 9th in a 12 team league (standard scoring) ... you might want to pick the Stud WR that hasn't been drafted yet, knowing that on your next pick your starting RB won't be that much worse.
In addition, since most scoring has Receiving and Rushing TD's marked the same, you'll probably get as many or more TD's out of your stud WR than your middle 2nd tier RB. Yardages are only a point or 2 here and there, so if the RB rushes for 1200 yards and receives for 200 and your WR receives for 1200 yards, it's pretty much the same since most leagues only give bonus for long rushes/receptions and how many 20+ yard receptions will an RB get ... so the bonus at this point doesn't make much difference.
That said ... standard draft format of RB-RB first isn't a terrible way to start ... but if you think you can get approximately the same value with the next round by waiting 5 picks, then you might consider drafting a top tier guy in the other major position.