I dunno why you are so obsessed with this. We have seen both Phillips and Allard show signs of being mirages, and Sanchez is struggling too. They need him, and they think they have solved his issues (both his velocity/grip and his injury). They may well be wrong, but the guy was our one of our Top 3 starters last year (second in WAR, first in wins). If they needed him to be that - or even a #4 - this year that would be a problem, and it was also not ideal that they handed him the #5 spot when they did, though I'm still not sure Turnbull wasn't a mirage as well (at least he is health-wise).
But now, it's a no-brainer. They need him to eat some starts. They need to see if he is fixed, not just for the next six weeks but next year. Whether he's 6th in a six-man or just making a couple of starts until Suarez comes back, he's the right guy at this time. And he'd otherwise be taking up a spot in the bullpen where he'd either be unsuitable or would just end up soaking up innings after someone else's disastrous start.
And of course it's also about the money. But I don't think they'd be doing this if the Philliies were only two games up in the East (if that were the case they might have made that trade you wanted so badly).
Right now every Walker inning is just a about saving some better innings in October from someone not named Walker. Even if he goes 0-3 (but this offense, while not exactly up to the task, should be able to win a few of his starts like they did earlier this year).