I think the only thing saving Hammerstein are the balconies to be honest. Roseland only has one upper level which is GA. Hammerstein has actual seats on 2 levels. It's also perfect for sit down shows, not to mention wrestling and other special events. Also Hammerstein is bigger than Roseland capacity wise 3500-3200. There was really no need for Live Nation to have both venues. Hammerstein keeps getting the almost arena ready tours while Roseland keeps getting artists on the way down.
bingo, live nation doesnt need 2 venues in the same capacity range, although important to note is that Hammerstein is now an open room that any one can book (Bowery just did alt-j there).
Also interesting is that I hear Roseland and Hammerstein are both expensive rooms. take alot to run especially compared to Terminal 5. (one of the reasons that venue is probably still around despite its terrible reputation among new yorkers)
Honestly for me, with a few exceptions, past Webster Hall/Irving the shows tend not to be as good. Central Park and their Pier were good, and ive seen some amazing shows at T5 and MSG, but by and large it seems tougher for bands to deliver.