I'm afraid Williams have never intended to maximize Senna's potential and always treated him as a temporary solution. I think it was a part of a long term plan.
Let's look at it:
- Williams had poor 2011. They needed to rebuilt the team. But they needed money to do this. They were lacking a bit of cash after placing themselves at 9th in WCC and losing title sponsor without replacement of it. They needed to make themselves able to build a competitive car again rather than having two competitive drivers. They needed money with an immediate effect in order not to go further with their crisis of performance.
- So they went for Bruno, as it was probably the most efficient way (mabye the only possible one) to gain the amount of money they needed. Moreover, they could force him to share the car with Bottas, as Bruno had no other reasonable options available. That allowed them to gain another $$$ (not very much though) from Bottas' personal sponsors for giving him that much time in the car.
- They've rebuilt the team before 2012. They get "a one off payment following a new commercial agreement for their continued commitment to Formula One" this year, and they will improve they WCC standing by 1 or 2 compared to last year. So they will be able to drop Senna's backing without that much negative influence on the team. And they will have Bottas, the driver who was probably intended by Williams to get a racing seat somewhere in the future. Bottas now has more experience in F1 car thanks to being that much in the car in the cost of Bruno. That will help him a lot.
So all in all, they've used Bruno in their plan to get money exactly when they needed it the most (winter 2011/2012) and to give Bottas more experience with F1 car making him more ready to be a future lead driver of the team. After this, they don't need him anymore so they will probably let him go. I would say poor Bruno, but it was his own choice to agree on Williams terms. And he could do yet another F1 season, which would be probably impossible without Williams. They knew they needed some kind of compromise in 2012. So they decided to go for an intermediate year with Bruno, with a risk of not maximising their 2012 performance, but with a better prospects in a long term.
I think Williams thought a simillar way that I've described. It makes sense for me. It could make even more sense for them.
This sums up the situation very well.
At the end of the day it worked out OK for both parties as Williams got the funds they needed to get a decent design team in place, and Senna got a drive when he didn't have any other offers. If Senna was fast enough the missed FP1's wouldn't have made that much difference. It's a shame as he seems a very likeable bloke and I would have loved for him to be faster than what he has shown so far.