I don't think the argument is that rookies can't, under any circumstances, play big roles on contenders. At least, that shouldn't be the argument, in my opinion.
Rather, in a draft like this, I'd say it's almost impossible to predict what players could play a role. Tons of rookies -- even lottery picks -- look lost their first year, and often hurt their teams more than they help. They're essentially getting on the job training, that doesn't necessarily fit in with being a contender.
When I'm assessing a "win now" team, it's hard to peg a rookie's value. Usually with rookies, the downside is as realistic -- or moreso -- than the upside.
Since they're the subject of today's debate, I like both Greg Monroe and Ed Davis, especially Monroe. I think both will turn out to be good players in the NBA. However, if I had to assess their chances of both being championship team rotation caliber players right now, I'd say the chances for each individually was below 50%, and the chances of both being of that caliber in their rookies season would be below 25%.