What's the deal with prime lenses

Primes are smaller and lighter. With the 18 or, the coming 27, the xe1 is X100 small. I tend to carry that package with me frequently. The zoom I'll use for travel.

For me, with the Fuji zoom, the optics are secondary. With the ois I get the stops I need for low light and the optical quality is good.  So it comes down to which lens am I willing to carry more often.