This is a stupid topic (no offense to the OP). Why should Western Union and Moneygram use Bitcoin, when it is a competitor to their business. If people start using Bitcoins for remittance, then the revenues for WU will drop
If you can't fight with it, try to embrace it
For example, this is what Microsoft has been doing all their history. Why should WU be any different? These are ruthless businesses which don't care about changing their stance 180 degrees any moment and all of a sudden accepting things they have just been condemning if doing that helps them with their profits. So just being a competitor doesn't mean that they might not embrace Bitcoin in the future. In fact, such things happened many times in the past and continue to happen every other day
I agree 100%, WU or Moneygram can embrace Bitcoin and use it to transfer money cheaply around the world, or they can keep on spending more than they need to.
Private persons probably won't move money internationally very often using Bitcoin, but a new company could move in and undercut the established companies by using Bitcoin.
That would be reason enough to start using Bitcoin, I think they are basically ignoring it at the moment as they would have some liability to the price, and there would be a set up cost.
They will do it sooner or later though, or someone else will.