I agree with you.  And it seems like it would be an easy thing for them to add to the list of things we can choose to display in the client list.  It would be good for you to post this in the idea suggestion area.

In the meantime there is a work around:

On screen 1, towards the bottom, in Miscellaneous Info, there is a field called Custom Filter.  You can use this for a promise date.  Then go to Settings>Options>Display and in the Available box open Miscellaneous and select Custom Filter. Then click the > to move it to the Display box where you can use the Up & Down keys to position the column where you want it.  The heading will be Custom Filter, but I think you may still be able to click on it to sort.  Not sure about that thought.