So BLM as AOE covered, MNK has single target... What does SMN bring?

[Dyvid]

It's a lot more complicated than that, because most suggested buffs to SMN (Aetherflow, MP Costs, Potency increase, cross-class Thunder, Energy Drain buff etc.) would all affect SCH too.

SCH's DPS is already very potent within endgame, all the while handling their healing duties and being able to take care of additional mechanics such as T12 Bluefire.
Buffing any of the above-mentioned things would be tipping the scales of healer balance further and further in SCH's favour.
This isn't an issue now since healer choice isn't as competitive as DPS (2 healer roles, 2 healer raid slots), but consider when Astrologian is released, it'll be more competing for WHMs spot... not SCH's.

They have to be really careful with how they handle changes so that they only buff SMN alone, and this pretty much limits the possible choices to things such as pet potency/attack speed or Enkindle/Fester changes.

This is true to a point. They could add / change SMN traits and abilities without messing with SCH. If the issue is mana then don't really need a potency change on abilities. For example addiing to Spur : pet damage restores player mp. Another would be removing enkindle, add a fifth pet action for inferno, aerial blash, earthen fury, then make the fifth SMN ability something like Aether Siphon that drains pet hp to restore mp.