From a Lechmere station discussion in the GLX thread:
Study after study has shown that putting pedestrians up on the bridge is a loser--what the 1950s thought the 1980s should look like. Overpass thinking led to downtown hamster tubes in Minneapolis, Cinci, Atlanta and they've since been acknowledged as an urban-design disaster.
I have no idea if this is true or not, but back in college my friend from Minneapolis (who, incidentally, was really was a major factor in getting me interested in urban stuff) would often talk about how the Twin Cities were the only major city that frequently had to broadcast hypothermia warnings, and then would talk about how there were more skyway tubes than any other city, due to the cold (not anything anti-urban). Not gonna bother to investigate the truth of that, but that's what he used to say. It's probably both, since it certainly is cold af there.