And the fact that Purvis and Wade have pulled out of writing more Bond soon, IMO, hints at their understandable displeasure of remaining the whipping boys for everything that is not good enough, while the script doctor or director-associate (Haggis, Logan) gets all the glory.
I believe it´s just the other way around and that P & W actually have contributed most of what was great about the past Bond films. Sure, Haggis and Logan may have added interesting dialogue. But how much of that was really their idea - and how much was influenced/worked out by the director, the producers and the cast?
Now, this is only speculation, of course. But I could imagine very well (especially with Logan and Mendes off to do their tv series) that the next director will bring in his own favorite writer and Logan will step down.
You're probably spot on. I still feel bad on behalf of P&W that they get so much blame for QoS when we know there was a writer's strike, and Craig and Forster have stated they wrote a lot of it as they filmed. Somehow, Haggis seems to escape being blamed for QoS, yet gets lots of praise for CR.
As for Logan's significane on SF, the Empire podcast last November with P & W pretty much says they did most of the work on SF and Logan's contribution was...limited. Also, I remember Logan saying somewhere that Bardem (like many good actors) liked to improvise and play around with his dialogue and came up with some great stuff.
Admittedly that should all be taken with a pinch of salt: P&W are naturally not going to talk themselves down and EON have hired Logan for a reason, presumably - but I wouldn't be surprised if Logan was hired because they wanted Mendes to return, and those two worked out the story for Bond 24 together, according to P&W.
It seems very unlikely Logan will remain the only writer on the next two Bond films. We all know these films go through a multitude of writers and script doctors before the final version is credited to two or three names.
I always assumed that P&W were EON's in-house writers from TWINE onwards, working out the plots and characters for each film over many many drafts, until a final few other writers were brought on to polish things up, add sparkle, etc. Directors on these projects nearly always bring on their favoured writers to work out the shooting script, and like you say they usually do the bidding of the director, 'correcting' things the director doesn't like, developing story and characters in ways the director prefers and, I assume, adjusting the tone to suit the director's preferences (although the producers are unlikely to hire a director who doesn't accept the tone and story of the draft they like). Then when it comes to shooting, of course, there's the influence of the actors, cinematographer, 2nd unit etc. It's all a melting pot.