A little, is still better than
nothing.
And I'm not entirely sure these drivers are as simply "arrive and drive" as you put it. Much understanding is required with these "spaceships", how the ERS power is generated and how long it can be deployed. For instance Button during many interviews last year was quite outspoken about what they were lacking and what he felt other teams are doing better. This tells me that these drivers do have quite a broad understanding on what is going on under the shell. They must, in order to extract the most of it.
Sure, I don't expect a Wehrlein or Haryanto to fully understand. Or even Verstappen. He probably doesn't have the understanding quite yet and also lacks the experience of having driven different cars to have any sense of comparison. But the drivers we are talking about here, Button or Rosberg, have clearly been in the sport long enough to have very insightful information about what their teams are doing, even if they may lack black & white numbers or a very deep technical understanding.
Also; You quoted Hill and Williams. Why go that far back? 2007 - McLaren and the whole Spygate shows pretty clearly that Alonso had a very good understanding of what was going on on a technical level that he then later used in the whole blackmail. This doesn't suggest these drivers are simply "arrive and drive" at all.
Back to Button to Williams; What we are seeing with Williams falling back IMO has a lot to do that the Power-Unit advantage is getting smaller. Mercedes, as the factory team, might still have a edge, thanks to insight into their own engine and also the engine maps and the fuel - something Williams can't quite exploit, but in regards to aero, even in 2014, I believe they were quite a far bit behind and that was only masked because back then, and in 2015 too, they were still riding on having a large engine advantage. I'm not suggesting that a driver like Button could come to a team like Williams and turn things around - not at all - but he might carry some interesting insight and knowledge that could help point some engineers at Williams in the right direction where some of that [aero or chassis] deficit may lie.
And if you have two drivers like Massa and Bottas, neither really destroying the other and the former not really being considered "top of the crop", well, maybe a driver like Button on a year to year contract might be a worthwhile gamble.
