I don't know that I'd totally write him off. I'm using him for the injured Marcus Giles myself as well, and any player with double digit HRs and 2B eligibility sounds like a viable option to me.
Between Mackowiak and Uribe, my MI has 4 HRs in the past two days.
Actually, I liken Mackowiak to last year's Frank Catalanotto, not so much that they are similar players, but because he has 2B eligibility, which Catalanotto had last year. While Catalanotto was a consistent player with a good BA, occasional power and deent run production, Mackowiak is going to be a pretty nice power source, but with a slightly lesser average.