Lore wise, and if I remember what Y'mitra said correctly, a summoner can only be attuned with 3 primal/egis at a time. This, however, leave a little loophole that will be used for the future egi glamours. It says only 3 at a time, but that means that, as long as we don't try to attune with a 4th, we can switch with any 3 primal/egis we have defeated before.
At least, that is what makes the most sense lore wise with regards to the egi glamours that are coming eventually.
If you have to ask, why only 3, its because more would cause lots of shenanigans to the summoners health and aether levels and whatnot.
On a side note: Warring Triad egis for 4.0 please!
