Donaldson is a nice signing for them but I just don't think the Phillies see 3B as a priority. Worst-case, they've got Franco. Then Santana. Then Crawford. Plus any number of secondary FA options if they do in fact sign Machado (and he doesn't want to play there at least part of the time). Even at the higher level I might prefer Moustakas (who bounced back defensively), and Boras probably won't let him get locked up quickly.
Yeah but his main value is his ability to play positions the Phillies don't need to cover. Especially if you sign Machado because then Crawford goes to the bench too. And if he's your primary offensive bench piece you can do better with an OF (or Marwin Gonzalez for instance).
But I also don't think Dietrich is going to find many starting jobs, if a team wanted him to start for them they would have gotten him on waivers and paid his arbitration raise. Ditto Bour (though he'll have DH options).