The flip-side to that argument is that being honest about your intentions keeps you honest about your ability to deliver value. If I say I'm thinking about leaving and the company doesn't struggle to sort out how to make things work, then either we've agreed there isn't a future for me there or I just haven't been delivering enough value.
Pleasant loyalty is a good way to stay employed when you're doing a mediocre job.
So I like being ruthlessly honest about this stuff. I'm much much happier this way than I have been in the past where my relationship with my employer involved more charades around future plans.