It's an interesting thought. I've been on the both sides.
It is probably important to discriminate concerning positivity for the sake of glossing over the negative, and positivity for the purpose of encouragement, edification and giving hope when spirit is failing.
In a leadership role it is vital to maintain a vision that inspires and edifies. People can be inclined to create fears and anxieties that become self fulfilling prophecies. Often they have no basis in fact. In a goal orientated environment, solution focused thinking is paramount.
I get your point, but wouldn't you agree that an honest leader is better than one who paints a positive picture just for the sake of, and doesn't shoot straight?
Not that it always has to be that way.
I've watched a documentary that described one of the traits of leadership as the ability to lie convincingly. I found that interesting. It seems imperative that our leaders are able to put a spin on things to motivate us.
It is very counterproductive when you are, for instance and staying close to home, managing a group of engineers whose job it is to know exactly what is going on in a project. Realism is appreciated, motivational talk is laughed at, as most are inherently motivated to do as good a job as possible anyway.
Most engineers I know only stop giving a shit when they have become convinced on technical grounds that something will never work, yet are forced to continue. No amount of motivational talk will cheer them up, they will just get more annoyed, and you can forget about the coveted happy smiles all around.
Again, I'm speaking from personal experience only, I haven't researched this.
As an anecdote, I remember a project being named "Marvin" by its project manager and engineers. Management didn't pick up this was a reference to the depressed robot from The Hitchhiker's Guide to the Galaxy. It's the sort of too-subtle protest people are forced to use in an environment that thinks positive thinking conquers all, and blunt realism is frowned upon for not being cheerful enough and can even get you fired.
I'd agree that empty rhetoric is counterproductive.
My experience is that negativity infects a group if a leader is negative. If the leaders standards drop, the whole group lowers its standards. Pretty soon, nobody gives a shit. :-)