I think they had a better choice of proven drivers with Sainz/Hulk. Williams seem to be towards the bottom of the barrel on the driver market so probably saw reason to take a punt on him. I'd be surprised if there weren't certain performance clauses in his contract to insure them against him having issues over the season. Doing weekends consistently over the course of a year would be very different to the odd FP1 or moc-race run in a test session.
I don't think Williams are necessarily in a better position from a development point of view. Kubica has experience, but from a long time ago and not in the current generation of F1 cars. Williams should have accepted Ocon on a 12 month deal instead of Kubica. Strong Mercedes ties and a couple of years of recent F1 under his belt. They have Russell to invest in for the future, Kubica is a risk both short term and long term. For the second year running they have an unproven driver line up. Kubica, in my view, effectively enters F1 again as a rookie.
I think we have to judge Williams line-up by their situation. They're the last place team, hiring the best two drivers available never happens. In context, it's a pretty strong pairing. Kubica at least certainly has the potential to be good (As opposed to likes of Stroll, Sirotkin, Erricson, Haryanto, etc) I understand being sad about Ocon, but he never had a real shot. Mercedes give an engine discount instead of payment, and it seems that offer doesn't stack if you take two drivers. Even those not using pay drivers wouldn't hire Ocon. McLaren and Renault passed him over because they can only guarantee a one year contract. As for why Kubica might be better now than 2017. Remember that both Renault and Williams were quite enthusiastic about him in their old cars. The Issue may have been adapting to the new regulations rather than his physical limitations (something a year as development driver can put right). Also, according to a Mark Hughes article, i read this morning, even in the test that lost him the drive last year he was still faster than Stroll.
I wish him good luck, even though it creates an awkward situation for Russel. Russel has essentially an unknown team mate to be compared against. Hopefully they both have car to get points with next season.
Yeah, he's hardly blown Perez away on pace. Another Hulkenberg maybe. A lot of expectation but hasn't really delivered what was promised. Like Verstappen, he also needs to avoid other cars more if he expects to get better, gest into too many silly scrapes.