­Glorious Anthem effects were traditionally rare. There have been occasional dabbles in them at uncommon - Celestial Crusader, Kabira Vindicator, Orcish Oriflamme.

The same is true of tribal pump effects: usually rare (Lord of the Undead, Goblin King, etc) but occasionally pushed down to uncommon (Daru Warchief, Imperious Perfect, two or three others).

I have actually playtested a set that had a Glorious Anthem at common. (One draft of Vitenka's dwarf set, perhaps? Can't remember.) It was just as format-distorting as you might expect. These kind of global effects are really best kept at rare by default, occasionally pushing them down to uncommon if you've got reason.

a lot of my creatures tend to be low-powered and/or high mana cost, due to galvanize and shabtis. But you're right, uncommon.