Supporters can only add to damage done by the party, or reduce damage taken by the party. That's all any role can do.

If they increase damage done by a greater number then the dps role that they filling, they are mandatory.
If they reduce damage taken by a great amount then a healer can heal, they are mandatory.
If they can do both, they are mandatory.

Either way, you are now forcing a group to take a supporter, becauses fights will be balanced around their presence. This is why we won't get the traditional Haste spells, because it would instantly be tagged as mandatory and balanced around.