If I Could Only Have 1 Mirrorless Camera?

Lots of reasons.  Maybe you want a carry a second body for backup, or had a second body for IR work.  Maybe you like the Fuji ergonomic experience and carry that around just for fun.

A smaller and lighter experience doesn't have to be as you described.  I used to haul aD2X, with a Fuji IR body, 3 f2.8 zooms, a 50mm prime, and an old 500/4 with a couple TCs in a monster PhotoTrekker bag.  It weighed a heck of a lot.  Now I use a Domke F2, which is half that size and weighs less than half of that.  In it I can have a hell of a lot, including an OMD, a backup body (I have been lazy and not sent off an old EP2 for an IR conversion), a Fuji X100, the Oly 12, 45, 12-50 and 75-300, the Oly 43 50mm macro,  and the Panasonic 20mm.  Sometimes I drop the Fuji and add the 43 Oly 150.

It may not be your small and light, but it is nonetheless very transformative.  It doesn't kill me when I walk through an airport.  I don't have to shove people aside in a dash to ensure I get an overhead compartment.  I take about 80% of that gear on long day hikes (think 10 miles up and down a mountain), and massively decrease my end of hike soreness.

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http://jcharding.zenfolio.com/