I played a semi-weighted MIDI keyboard for years, and usually play piano and organ sounds.
When my keyboard failed, I replaced it with a hammer action one and boy, was there a difference in what I heard from Piano synths.
The playing had so much more dynamics, and it was an audible difference. Even though the weighted keys are supposed to be harder to play, I actually found that it was harder and needed more effort and force to play loud sounds with the non-weighted keyboard, which is of good quality brand and make.
Also, besides the momentum mentioned, I notice by measuring that the keys of the Piano type weighted keys were longer by half an inch than the non-weighted and also maybe about 1 millimeter wider, so there is also a travel distance to consider. It is not the same to cram 128 values into a X distance, than into Y distance travelled if Y is larger than X. You immediately get more expression room.
I mention that on the first keyboard only a few keys failed, and so this comparison was done with both keyboard playing the same MIDI channel and the same exact patch and synth, with both connected and placed side by side.
I think is something similar to compressing audio. You compress audio, and you loose dynamics.
I even like the hammer action when playing B3 Hammond clones. One of them does respond to velocity sensing, another responds by making certain clicks and effects in accordance to velocity...So there IS expression there to be had too.
So if expression is dynamics, there is a definitive real, measurable difference in the distance travelled, where the 128 values are to be rendered.
In my opinion, and I have had various MIDI keyboards, and had to fix some and discard others, the best is yet to come when they stop using the silicon bubble switches to detect "velocity", and start using optical sensors.
I also would mention that by plain attention to the issue, we should realize that the velocity with which we play is not constant, but rather is variable, meaning that when we strike the key we can strike fast and modulate to soft and vice-versa. This is very difficult to formulate with only a starting time and ending time, as is currently done. So, a real piano, is way a difference in expression, and even more so in actual sound production, when compared to any computerized rendering, and hearing is the feedback to the pianist and is used to modulate the expression. We are fortunate to have the current technologies, though.