It is also counterproductive.
If you are not a junior employee, saying yes to everything takes focus away from the important tasks (not everything can be important).
You'd be better off carefully curating the set of tasks you say yes to, and ruthlessly executing those to perfection.
Otherwise, you will inevitably end up spreading yourself too thin, and doing a bad job at many things.
The colleague who always says yes doesn't have a boundary problem.
She has an identity problem.
At some point, saying yes became who she was. And now saying no isn't a decision. It's a loss.
That's a much harder thing to fix than a policy.