My opinion on the matter:
First of all, I fully agree with Hampus' argument that it's alright for a rookie driver to make mistakes as long as he's extremely fast, but not the other way around. In this regard, Maldonado is light years ahead of Senna. Apparently, sometimes even the Williams engineers don't understand how his qualifying laps were even possible... Add to that the fact that Senna is not even a young driver anymore, he's already 29. Also, Maldonado is always accused of crashing all the time, but frankly, Senna crashes just as much if not even more, the only difference is that his maneuvers are usually not as incredibly dumb as Maldonado's, so he only gets a penalty or loses a frontwing etc.
Considering this, I fully understand the decision of Williams.
However, in Senna's defense:
First of all, Maldonado's win at Spain was definitely extremely lucky. Williams just happened to understand the tyres very well during that race, and that's all that mattered during that part of the season. Second, McLaren screwed up. Third, Ferrari screwed up Alonso's strategy (if they hadn't he'd be WDC too now, btw.
). Without that win, Maldonado would only have 20 points, as opposed to Senna's 31.
Second, as stated above, there's the argument that rookies are allowed to make mistakes and that Maldonado can and will improve. If he's just immature, that's fine. However, if he's got an IQ of 90, which I suppose is equally possible, then there's just not much more going to come from him.
Third, the fact that Senna never got to participate in FP1. I know there are drivers for whom this doesn't matter one bit, but I think Senna is not one of them. In Hungary, he had his best race of the season, easily beating Maldonado, which, as he later said, was because that was the only race where he really understood the car (but he didn't really know why). If Senna was always this fast, I think there might be a future for him in F1. In a new team, with a different car and the possibility to participate in the FP1 sessions, who knows, maybe he's going to get that better understanding of the car.