It is the same argument for buying any device.
Ok. So you're argument is "1st party titles". Fair enough.
You buy it because it plays the software you want.
You don't say "I'm going to buy an X360 but no games". Instead you say "I want to play these awesome games so I'm buying an X360." Similarly for the 3DS or any Vita or any other device.
A tablet just has many more uses, and it has many more free and inexpensive apps. Even so some people have no inclination to buy one because it has nothing that they need or want.