The problem with what you're saying is diplomats are not dealing with "reality" per se, but rather with peoples' emotional responses to reality. Straight talk is not always the best way to defuse a problem -- sometimes it's best to defuse a situation by allowing the other guy to back off without losing face.
"Telling it like it is" certainly has its place, but it carries along with it an explicit threat, which can be reliably counted upon to provoke a "fight or flight" response from the other side. Too much plain-spokenness tends also to provoke a lot of unhelpful resentments from lots of other countries, friends and enemies alike.
Sort of off-topic, but I'm guessing that you work in a technical/engineering-type field of some sort. Am I correct?