I'm with you that the screen on a Retina MBP is a leap ahead, but I don't think I'd call it a game-changer.
For one, I do 80% of extended work sessions when I'm at one of two desks. At both those desks, I've got an aforementioned docking station sitting there, and plugged into each docking station is a pair of beautiful Dell 24" IPS panels. That's more screen real estate, and in a more organized and usable way, that I'll get on an MBP Retina. If I did most of my work without these external displays/setups I may feel differently, but for now, they come with my main use-cases.
When I'm not at one of those two desks, would a Retina display be nicer than what I've got? Sure. But I wouldn't trade it for the control nub, the user-fixability, the ability to send a hundred bucks to Crucial and get 16GB of RAM at a time of my choosing or order a second battery if I'm going to be off the grid for a weekend, the docking station, or the keyboard light.
Neither machine will be the ultimate machine for every person. For the way I work, the X200 is in a league above the MBP - the better screen doesn't make up for the machine's other failings. If you work differently, your mileage may vary.
I won't argue practicality or usability (a dual-monitor setup has distinct benefits, as does an increased physical area), but my computation says:
(2,880 x 1,800 = 5,184,000) > (2 x 1,1920 x 1,200 = 4,608,000)
Of course, you have to be crazy enough to run the MacBook at its native resolution without scaling to reap this benefit (I am crazy in this way).
