Basically, it's a cheap and easy way for Imperial armies to take a Priest without any additional "tax" so long as they have a Detachment slot to spend.
Warzone Atlanta had unlimited Detachments so you could use this without concern.
Adepticon Championships and Maelstrom, however, share a restriction of 1-2 Detachments. Needless to say, that 2nd slot requires a lot of thought!
I think ITC is sticking with 3 Detachments which grants a bit more flexibility though not a lot in this modern age of GW making many, many mini formations.
Sometimes it's amusing how the Tournament Scene is trying to clamp down on mixed force shenanigans by limiting choices while GW is trying to get people to buy as much variety as possible by making so many small detachments. GW's latest maneuver is the Castellan detachment which allows you to mix many sources in a single formation. Plus St. Celestine and Cawl counting as HQs for any Imperial Army. Brilliant!