many people are too eager to give out their cell number, put their work email on their cell, etc. I did that for a job once and decided to never do it again. It hasn't held me back to set up realistic boundaries but it has prevented them from bothering me when I'm not at work.
Now, if you are issued a company phone, or it's part of you job to be available all the time that's different. But the average office worker does not have that requirement, they do it to themselves to show they are a "team player."