Walker is a journeyman type who has had some initial success in the closer role. Same can be said for Turnbow this year and Herges last year and Biddle the year before. Despite his last few appearances, by all accounts he's the closer for now. Doesn't mean it will last, though. Regardless, should he struggle, I believe Hawkins will get a shot.
One of Hawkins big issues is the HR - and HR are harder to hit in SF than at Wrigley. Hawkins was also allegedly getting hate mail - which might have affected him. If he is under less pressure and in an environment more favorable to his style of pitching, he may very well have some success in the role - especially if SF is out of the playoff race (less pressure).
So, while I think he's set-up for now, he's a nice gamble because he's historically been very good as a MR and should give you good numbers even if he doesn't become closer. Not to mention, he has the chance to pick up some save chances should Walker fail to hold onto the job.