For a large consultancy with many clients, I agree.
But when a significant portion of your employer's income is a single client:
Are you comfortable with that single client of your employer having a strong incentive to make your job redundant so they can try to hire you under worse terms (as you suddenly lost your job)?
You don't need your company's permission, you're unemployed.
I guess what I'm thinking is something more along the lines of not hiring employees immediately after making them redundant through your own actions?