You might be confusing things here.
Dual channel is a mode that (most) RAM is capable of and that many motherboards (and CPUs with integrated memory controllers) support.
Triple Channel is not supported much and I'm sure you would know if your MoBo supported it already.
When you are hearing people say that more than two sticks is unnecessary that is because Dual channel only works (well) with Two (dual) Sticks. If you add a third stick you won't get the benefits of Dual channel as one stick will always be left out (so the mobo won't even run in Dual mode)
Quad channel (4 sticks) is supported by Servers (Dual CPU) and new CPUs that support DDR4 so...
If what you are really asking is why you shouldn't buy another stick for more RAM there are many reasons if you'd like THIS question answered