I think that Barrichello is allowed to compete. The rumors that he isn't allowed to race, or is being given an inferior car, just aren't true. He just isn't as fast as Schumacher, and never will be. There will always be the weekend or two per season where Barrichello will be unbeatable. Silverstone last year was one, Suzuka was another, and he would have done very well at Brazil, were it not for that "fuel pickup problem," which is F1 for "he ran out of gas."
It isn't Schumacher's fault that he's as good as he is, and works as hard as he does, nor is it his fault that there isn't really anyone of his caliber in F1 at present. Raikkonen will be there, as he is quite talented, no question. Montoya will never be there, ditto for Alonso. Drivers such as Webber or Button wouldn't be much better than Barrichello.
Besides, who in their right mind would want to partner someone that can beat them like a drum week in and week out? Barrichello is very fast, and very talented, certainly one of the top five drivers in F1. But Imola demonstrated why he just doesn't have the mentality to excel. Schumacher would have found a way past those slower cars, on the track or otherwise. Barrichello couldn't/didn't, even though if you look at his racing line compared to the cars he was stuck behind (R. Schumacher, Trulli) he wasn't using anywhere near the amount of track the other drivers were. He was definitely faster.
I think that the other teams (well, not McLaren...) will definitely catch up to Ferrari. Renault still needs a bit more power, and Williams needs a bit more something or other. BAR should do very well at Barcelona. Don't forget that they have owned that track all during testing season, etc. This could be Button's first win. Once somebody else notches a win, people should calm down.
But I think that talk that Schumacher is "holding down" Barrichello, or that Barrichello isn't being allowed to race to his full potential, is misinformed. Barrichello is the no. 2 driver at Ferrari, in that the team is set up to have Schumacher win. This is no different from cycling, or a host of other sports where teams have a no. 1 rider. Perhaps if other teams adopted a no. 1 structure, they would be more focussed.
I would further argue that, with his pace, talent and hard work at raising Ferrari up from pack fodder to excellence, that Schumacher has earned his no. 1 status. In thinking about this debate, I don't know if speculation about whether or not he is good for F1, even though everyone is having it, even at the Ecclestone/Mosley level, is fruitful. He's here, he's good, and short of the FIA adopting the "success ballast" of other series, his dominance is a reality.