The pros:
- More people owns a 3DS, thus the game will have more exposure
- The 3DS will have a system seller
- It creates anticipation (a.k.a. hype) for the "real deal"
- People who were considering skipping the 3DS version will have a reason to buy it
The cons:
- A few impatient gamers will skip the Wii U version and settle with the handheld version
- Some people will misinterpret this into thinking Nintendo consider Wii U second priority