It would not be a waste, unless you need to duplicate files all the time or need to have that fast write and read speeds all the time.
I have a Vertex 3 (S-ATA 6.0 Gbps (S-ATA III) SSD) in my 2009 MBP, which only has an S-ATA 3.0 Gbps (S-ATA II) interface. While I not get fast write/read speeds (I use another external HDD for storing big files or external storage), as I don't need them, I can still use the SSD and all its other features like fast random access times, which is more important to me than just the constant read/write action.