You find 1920x1200 less productive than 1600x1200?
In this specific case it is most probably rather a matter of visual ergonomics. If you compare the DPI resolution values of the following three common display types, there is much more difference than just added vertical screen real estate (DPI values on the far right side):
UXGA 15 4/3 1600 1200 133,33
WSXGA+ 15,4 16/10 1680 1050 128,65
WUXGA+ 15,4 16/10 1920 1200 147,02
Once i had a T61p with WUXGA+ (1920x1200) for a short time, which i finally got rid off simply because i found this high DPI value practically impossible to work with. In comparison, the T61 which i still have with its WSXGA+ (1680x1050) is very easy for the eyes thanks to a very good DPI value. In the end i settled for a T61+ Frankenpad with a UXGA (1600x1200) as a perfect compromise.
Leaving the added benefit of its Flexview characteristics aside, the UXGA DPI value is still rather easy on the eyes, being only 5 DPI steps away from the most comfortable WSXGA+ DPI value, while the WUXGA+ DPI value is almost 20 DPI steps higher and unfortunately has proven to be beyond reasonable ergonomic limits, at least in my particular case.
If we go a few steps further and compare DPI values of some other popular screen resolutions, there is no real joy anymore either:
SXGA+ 14,1 4/3 1400 1050 124,11
WXGA 14,1 16/10 1280 800 107,05
WXGA+ 14,1 16/10 1440 900 120,43
WUXGA 15,6 16/9 1920 1080 141
If one wanted to recover the vertical resolution of a standard 14.1" SXGA+ (1400x1050) screen, one would need to settle with a 15.6" WUXGA (1920x1080) screen with an almost 17 steps higher DPI value, which might already be too much for some people's eyes. For someone who values vertical space much more than horizontal one, this is indeed a bit uncomfortable to swallow.
So there is obviously a little bit more to the screen resolution changes of Thinkpads during the last years than just the display ratio, and this is in first place regarding DPI values. Young people with still perfect eye sight might find this to be no issue, but for me this does make a big difference.
Depending mostly on text oriented usage, fortunately, i am in the position to not rely so much on CPU speed and the latest graphics gimmicks on any of my Thinkpads. Therefore i expect to be able to use my wonderful T61+ Frankenpad for many more years to come until there is no other choice than switching to something more contemporary. Who knows, maybe this non-ergonomic wide screen craze will have made room for something more reasonable until then.
(Resolution values were taken from http://thinkpad-wiki.org/Aufl%C3%B6sungen