Apart from the weight (and perhaps bulk) issue, another advantage of a monocular is that --- for me at least --- it's a lot easier to carry it in a way that it's really convenient to access on the trail.

To be clear, I rarely carry one, but the times that I have, just having it very readily available makes all the difference. For me, that's typically attaching the case to a pack strap; it's light enough to do that, but even "pocket" sized binoculars are big, heavy, bulky enough that I wouldn't like to carry them that way. And they would take a lot (most or all) of the space in a waist belt pouch, which competes with other stuff I like to carry there.

Like a camera, for me it's either really easy to access or I shouldn't bother taking it, because it's rare that I'll take off my pack and dig around in order to look at a bird on a tree somewhere (or take a picture).

I guess it really depends on what it is that you're looking at, and how important it is to get the best possible resolution that you can. For me it's looking at wildlife (or sometimes "possible" wildlife at a distance) or scanning terrain features at long distances. A time or two I've been able to read a sign off a little ways where I wasn't willing to walk off trail far to see what it said, but mostly --- wildlife and terrain.
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Brian Lewis
http://postholer.com/brianle