May have wondered this before, but since newer information came out about upcoming events, it seemed a way to talk about it more.
So the information we have about mag'har orcs joining the horde is this.
Spoilers:
We're sent to AU draenor to recruit draenor orcs to join the horde. When arrive there its 30 years after the events of warlords of draenor, and in this alternative timeline (which would be in line with the current azeroth timeline, just in an alternative reality), the draenei lead by Yrel have used the light to bring orcs into their rule, either by choice or by force, making the orcs under their rule able to summon the light.
With durotan having lead a resistance against these draenei and their use of force to convert orcs to their fold, he was killed, and his daughter Geya'rah took over, leading the resistance. These are the orcs we bring into the current horde on azeroth, and guessing several of them (orc priests) were once shadowmoon orcs who the light forge draenei (Yrel's order) forced to follow the light, but joined Geya'rah's resistance and left to join the horde on azeroth.
This is a bit muddled considering, since this does actually give canon reason for these orcs to use holy magic, orc course like with a lot of upcoming allied races, having an orc like this, who was made a slave to the light by the light forged draenei, to now using that power against the alliance (but not before leveling to 100 and joining the priest order to help MU velen and other draenei who said orc should hate at this stage).
But, despite how muddled it all is, having orcs with holy magic (and shadow magic since they would have been shadowmoon orcs originally) does have at least some canon to it, more so then holy forsaken.
I'm so making a mag'har disc priest.