If Macias' multi-position eligibility isn't vital to you then yes, by all means. Giambi isn't a star (at least, this one isn't), but he'll be a serviceable outfielder, assuming he plays every day, as you said (I think he will). His stats so far are about as one would expect from him, although it's possible that he truly has gained a bit of pop. On the other hand, his numbers should dip slightly with the move to the NL. Where he'll bat in the Phillies' order will also be interesting.
At any rate, yes, by all means pick him up for Macias.