I wanted that option too. I can work out a justification for taking it on most characters I've tried out, but the one or two really selfless ones would have turned it down if they could.
My character viewed it from a pragmatic angle, without the ring mindflayers would likely be drawn to the location and regardless of whether Omeluum chooses to stay or flee they wouldn't be able to help my character further down the line. A friendly and helpful mind flayer seems like a really handy fella to know when your party's day to day life revolves around cryptic illithid tadpoles in your brain.
