"We had a deal! What about honor among thieves?"
"Oh, what a romantic notion.."
As a phrase, "honor among thieves" is complicated. As an ideology, it's even worse.
While there is a slight bit of divergence with this phrase and one the parallels it, "no honor among thieves", they can still both be used with the same basic meaning in situations of differing mechanics depending on the pro- or antagonistic bent that is being sought.
In the first usage it is commonly employed as something close to a modern day "professional courtesy". The idea here is that while a thief may lie, cheat and steal from everyone else in Gods creation, they will respect promises made to other thieves because they see each other as kin. A thief may not trust a Priest or Barkeep, but he'll trust his own. (Q: Why don't sharks eat lawyers? A: Professional Courtesy.)
And yet, on the other hand it can be used in the same manner to express the exact opposite feeling by employing those most wonderful tools of irony and sarcasm -- You expect a con-man to not con another con just because they're both cons? Honor among thieves? As if!
If you're going to plug in the "no" in the begining, it is almost always negative useage this way, to illustrate a particular character. Telling someone "there is no honor among thieves" is an esoteric way of telling someone to watch their back because if someone is intent on stabbing you in it, who and what you are won't change their mind. (A good corollary to this is the story of boy who climbs to the top of tall mountain and encounters a rattlesnake there, freezing to death because of the cold. The snake implores the boy to tuck him inside his coat and take him back down to the base of the mountain where is it warm. Yet the boy refuses saying "you are a snake, and you'll bite me". The snake promises that if the boy take him down the mountain he won't hurt him and so after much bickering, the boy agrees to ferry the snake down to where it is warmer. When they get there, the boy removes the snake from his coat and places him on the warm ground whereupon the snake promptly lunges and bites the boy. The boy is startled and pleads "You promised!" to which the snake replies "You knew what I was when you picked me up.")
But, back to the first version and the title of the node. As quirky and perhaps obtuse as the concept seems it still happens all over, though we seldom recognize it in practice -- we will be perfectly reserved to bullshit Joe Q. Public because they don't know better, but we're less likely to do it to someone in our own field because we know they have the capacity to call us on it. In that case, we're more likely to check our figures and make sure what we're saying is true -- not because we have any more respect for this person, but because we know our moxy won't fly. A voluntary double standard based around the amount of stuff we can get away with.
As well, if you constantly break agreements with your constituents (the people you work with), how likely are they going to be to trust you in the future? And if there is no guarantee that someone you know isn't going to renege on a contract, how likely are you going to be to extend it to them? It's also a way to look out for your livelyhood. It can be seen and a modified and selective version of The Golden Rule: instead of "do unto others" it becomes "do unto your own kind because they're holding the rope".