Undergoing the severance procedure and having your "innie" work at Lumon is exactly akin to having your own personal slave. The innie does all the work, the outie collects the paycheck. The outie controls whether the innie lives or dies, since the innie literally does not exist outside of work.

I guess another part of the tradeoff is that you're giving up a significant part of your life time-wise. The number of waking hours experienced by the outie would be substantially reduced (presumably around 40 fewer hours per week). Overtime would really suck, especially if unpaid. From the perspective of the outie, working overtime would be exactly equivalent to the company taking away additional hours of their life.