Noone is twisting your arm and forcing you to put a large lens on a Nex camera. It's flexible enough to allow it as an option, though.
The kit lens is a good all-purpose lens. Not the best, but pretty good. I get good sharpness -- I have to pixel-peep to see the difference with primes, much of the time -- and the bokeh and color is great. It is convenient, though. It's weak at 18mm, wide open... If I have a need for it, I'll break out the 50/1.7. I've occasionally adapted other lenses, but since small size is the Nex's forte, I tend to want to use few, and smaller, lenses.
The A900? That's large, even for a DSLR. I wouldn't get the Nex if you didn't care about size and weight.
I bought an old MD mount 135mm/f2.8 lens, which is not too large for what it is. I would think to get much smaller, you would have to go with a camera with a smaller sensor. I'm trying to decide if I can come up with a good use for this lens. LOCA is a bit strong, and, of course, I have to manually focus. There may be situations where it would be helpful to use this. But most of the time, I'll carry smaller lenses.