bkeich (and many others, to be fair) just finds Walker particularly triggering, for understandable reasons, but it's just as understandable to see the logic in using him as long as they have them (and with the sliver of a chance that he's okay).
And because the other options aren't better, than Walker or Schwarber. Case in point, spring training phenom Nabil Crismatt giving up six runs in four innings against AAA hitters yesterday. And we already know Johnson and Mercado aren't currently better. Plus they'd need a roster spot (even if that was just Carlos Hernandez).
The other option would have been Ross but I think the injury to Strahm made them more reluctant to burn someone they think can really help the pen.
In the end, if he eats inning he did his job. Phillies actually won four of his first five starts last year by scoring a lot of runs (and the 5th went 11 innings). Then they lost the last nine in row scoring four runs or less in eight of those (the best of which, for Walker, was the 6-5 loss in London to the Mets where he only allowed two runs).
And if he stinks, it's the same bullpen game they would have had without him.
