Last year they had more options for starting pitchers, and Mitre was pitching well out of the bullpen, so they chose not to move him out of that role. And yeah, if there are better options, obviously they wouldn't use Mitre in the starting rotation. Just like if they had five pitchers better than Phil Hughes, they wouldn't use him as a starter either. It doesn't mean they don't think Mitre can start, it just means they had better options.
The reason I bring up Wakefield's age is not to suggest that he'll be injured, but that the grind of a MLB season for a 45 year old might just be too much for him, even if he's not on the DL with a significant injury. All three pitchers, Wakefield, Garcia, and Mitre, are flawed, and I just don't think one is really a good bet to perform better than the others. But we'll find out soon enough.