When I did T9 (as a WHM) I was told it's better to push the phases so you don't get too many meteors on the field and that's why people prefered to replace a tank with a DPS.

Having done the fight with both 1 and 2 tanks, it takes a bit of practice to switch over.

For example, I was used to only dealing with one person to stoneskin for ravensbeak so that person has two dedicated healers. (but when you have two, you have to split duties)

On the flip side, with two tanks, if one dies, the other can pick up the pieces and continue.