The entire first page of the Bungie forums is full of threads arguing over Icebreaker. It seems pretty split between those upset over it being sold (and saying people didn't "earn" it) and those thrilled with it being sold.
As someone who got my first one from Xur (but then later had one drop at Atheon) I'm in both camps

.
Now, Gjallahorn - that one I don't think he should ever sell again.
I also didn't realize IB could be used to generate ammo for other snipers but I had this happen when I was running the Weekly last night. Of course on the Weekly (as opposed to the Nightfall) you can just run out of all your ammo and get an automatic refill. But one time I switched to IB when I was out of just special, then switched back to Black Hammer after a bit, and I had 3 more shots for it. That is something I may use in the future.
Black Hammer is just awesome on that Walker though - You can fire continuously on the legs and then on the main critical spot when it opens up. It makes taking that down much faster than it used to be, even with Icebreaker.