The trend is that airlines are less likely to do this. HA stopped doing this, and DL followed shortly thereafter. UA is still doing it, AFAIK.
That said, like many things, it's not as easy a with SHARES as it used to be with Fastair, so it's not surprising if you encountered some resistance from a harried agent. They could be frustrated that it's so much more difficult to do (ex-UA agent) or simply frustrated because they're busy and have to spend time doing something like this (ex-CO or ex-UA).