If you're mission is to run trails with a specific group of people and that group chooses to standardize on GMRS then it could definitely be a great solution. The range and clarity is by far improved over 5w AM CB comms. But its limited in features vs. a Ham. If, for example, your mission requires the radio to serve as an emergency help tool on a remote trip then GMRS isn't the best solution. Yes, you can supplement with a satellite messenger (which I do) but that tool has lag which a radio doesn't. A Ham will get you the best of all worlds for a small investment of time. Though, as you stated, if you run with a group that only uses GMRS and it solves the majority of your comm needs then it can be a great tool.
I, OTOH, almost never do group runs. I'm not a 'crawler' or OHV guy. I like to get away from people and explore. For me the radio is both a means to communicate with one of the few vehicles I travel with and a way to tap out to another human if help is needed (i.e. via repeater, 146.52, etc). It also offers the APRS technology which passively limits the SAR radius should I become incapacitated (which satellite technology does as well, though with my subscription plan the beaconing is less frequent and the dots farther apart than APRS). My cohorts do also value the Ham system so it's an easy one-radio solution for me. There's also the 'MARS' mod available for Ham that gets you access to the otherwise 'locked' public safety frequencies. While I'm no longer an active reserve deputy nor an active SAR volunteer I would have zero hesitation using a public safety freq for emergency purposes. So for me, the Ham is the best solution for my needs. And I don't want to get into a two-radio (or three-radio) solution space. I've ran Ham and CB before and it makes for a cluttered cab.
And just to clarify, GMRS is more of a FCC registration than a license. No knowledge testing required.
Lastly, and this is a grey area for some (and black/white for others), you can get some Hams to transmit/receive on FMS/GMRS bands. For example, my little Baofeng UV-5Rs that I keep as backup/spotting radios will work very well with my son's FRS. Just not legally. But at the mid power setting on the UV-5R it's very unlikely you're going to cause any real-world disruption on that band.
