I'm not so sure I trust those who are phlegmatic... at least not to an extreme degree. I mean I suppose it is possible to have complete control over your emotions, but is it possible to be completely unemotional? I think I'd rather not be anyways. I enjoy my emotions, even the wild ones. I feel like it's part of what makes me human. I know I'm not perfect and I'm okay with that (mostly).
For me, it just seems like those who manage to keep a completely straight face, have something to hide. I realize this is nothing but an opinion, but a phlegmatic composure is often times accomplice to a lie, or at the very least withheld information. Which in a sense is just a fancy way of dressing up a lie.
Then there are those who require this ability for their profession (police officers, psychologists, even doctors are a few that come to mind). However good these people's intentions are, there is a degree of secrecy that is required in their profession, thus the need for being phlegmatic arises.
Personally, I'm the farthest thing from phlegmatic. I don't think I could hide a single thought or emotion that crossed my mind. But I suppose that could be a good thing, at least I'm easy to read, and I'm a horrible liar.