Pretty much the first thing that came up when I Googled the MD was this photo of a set of them on an ICE:
These seats are incredibly uncomfortable, the Grammer ICE 3000 is better in every way. Who knows why they chose this piece of garbage design.
www.flickr.com
On which the photographer says the following. Though I've sat in one and I thought it was OK.
I'm sure I can take a stab at answering his question, though: "because it was cheaper".
To be fair, since that journey, I have discovered that the airline style seats are actually better than the table seat I had on that trip, due to better recline and adjusable thigh support.
Overall makes it quite comfortable, and the ICE version (even in Second Class) is absolutely leagues ahead of the apalling UK version, thanks to the recline/thigh support, better cushioning, padded armrests, the headrest cushion, footrest, etc.
Image of the fully reclined seat:
There are other countries that use them in Second Class too, namely Spain, Austria, and Uzbekistan. I believe the UK is unique in reserving the seat exclusively for First Class use (bar the sleeper, of course).