With IWT/CTM, you don't need to be on the tank (or any of the melee toons), to have them move into melee range.

However, as far as positioning the mob, or using the tank to face a boss away from the group... that is quite hard to do, when driving with the healer or one of the DPS toons. And it is very easy to do, while playing the tanking toon.

You could easily use IWT to move the tank and melee into position. Then PiP swap to the tank's window and turn/position the mob, then PiP swap to another window etc.

Once in position and with the mob facing the correct direction, being on the tank or not is irrelevant. But you'll want to be back on that screen if you need to move or turn the mob.

Being on the healer is largely irrelevant, thanks to repeater region heals or to a lesser extent thanks to several toon specific healing macros.

I could see you wanting to be on a ranged DPS or the Healer toon, to have everyone but the tank move with you, to get outside of some crap on the ground or a nasty AoE.

CTM/IWT gives you a bit of flexibility in that regard. You would need your set up to allow for a switch, without follow automatically occurring. If follow was entirely manually done, then a toon window switch would not mess up your positioning, and this would give you ultimate flexibility in how you play.