It's not a 100% absolute necessity to have a top RB in a dynasty league, but it's really nice to have one, as most of the champions either have a top RB or decent RBs and are stacked at other positions. It's rare to win any fantasy league without having at least good production from the RB spots. When I won my 8-player keeper PPR league in 2012 (behaves a lot like dynasty), my RBs were Rice, Richardson and Spiller. That was a huge advantage over anyone else at the RB spot (one of these was a flex).
It's easy to ignore the RB position in dynasty, since stud QBs and WRs have longer lifespans. But in dynasty, you really should only be looking at 1-2 years down the road, becuase the NFL changes so fast. No one can predict what's going to happen more than 2 years down the road. So while it's nice to have Andrew Luck locked up for the next 8 or so years, it's getting an elite top 5 workhorse RB for the short term is the way to go to contend for titles now.
So you were lucky in 2012 and all the stars aligned and all your RBs had great year. Probably felt great going into 2013 as well. How'd that go for you with those RB's in 2013? How do you feel about going into 2014? Spiller, Richardson and Rice would be great RBs this year if that was the weakest point of your team, but not if you drafted them in years when they were considered top 5-10 dynasty RB's and you blew your early round picks on them.
You don't realize it but you're arguing the opposite point. Imagine if all the stars didn't align for you in 2012 and only 1 or 2 out of 3 of those guys hit? You didn't win "now" and your RB group is most likely in shambles and you have to rebuild again.
That's why you don't build dynasty teams around RB's. You build them around other positions and wait for the stars to align.
I'm in a league that did a startup this year and my starting RBs are most peoples RB3's, but my WRs are Cobb, Floyd, Evans, Patterson, and Hunter. I also have Graham at TE. I did a lot of trading to get my team like this, but I also acquired a bunch of rookie/questionable RBs like David Wilson, Andre Williams, Freeman, Bryce Brown, Donald Brown.
I'm hoping that after a year or two at least 3 of my WRs are studs and I'll have Graham at TE. Then all I need is one of my young/questionable RB's that I plan to continue to load up on each year will hit and I have a championship team.
If I grabbed RB's early, even if they are young like Martin, Bell, etc, you are planning to win now even if you don't think you are. Guys that are perceived as young stud RB's rarely become the next Adrian Peterson who plays at a high level his whole career. Most guys get passed on the depth chart sooner than expected.