I don't think it's aesthetics or even safety that the concern. The fact is that the MTA has huge budget deficit that is growing by the year and a pretty substantial amount of debt to go with it. I know they were trying to levy a large amount to pay off their debt against property they own on the west side of Manhattan, which I'm not sure they ever sold (I don't know if anyone remembers when there was talk of a new NFL stadium possibly being built in Manhattan). Regardless, they are way over budget already on their current projects (a new line is going in on 2nd ave and other station improvements are ongoing) and the flooding from hurricane Sandy certainly didn't help things (draining and cleaning the rails of the corrosive seawater was a huge ordeal). To my knowledge, there are two stations that have rails, one being the South Ferry station, where the track and platform curve around, as that is the end of the line in lower Manhattan. Either way, it would be a huge project to install anything like that at every station (over 400 of 'em) and there would still be gaps in between the rails where the doors open to board the subway cars. I remember it was a huge deal during the Giuliani era when they installed little dividers on the wooden benches so that homeless people wouldn't be able to sleep on them at night (I always thought that pretty callous). I wouldn't be surprised if that project alone cost the MTA 10's of millions considering their record of inefficiency.
