I don't see any harm in having this as an option.
There was a thread not so long ago asking people if they farmed mounts on their alts and the conclusion seemed to be 'no', why redo old content simply to get a reward you already earned when you could be getting something new?

My feeling is this, I did the work on my main so I'd like to have the rewards on my alts. People who want to keep each alt separate should of course, have the option to do so.