These add-ons are completely unnecessary.
Unless you want to play actual tennis with Wii-motes or something, then you would look like a complete douchebag using this stuff. Don't mean to sound like an asshole or anything.
Would people really buy this stuff though? Well, maybe that 40 year old mom I saw at EB buying Game Party or something for Wii would buy this lol.
It is pretty neat though.