They will just add to the clutter, congestion and disarray in Manhattan. Ugly concrete barriers. Adds to the Third World look and vibe of NYC.
Whoever makes them is getting rich.
As a regular bike rider in Manhattan, it would make things a whole lot safer for bicyclists, and for pedestrians, who often blindly walk in or cross bike lanes, usually while texting. However, as you say, they would look awful and be very expensive, given that we have so many bike lanes throughout the city.