However, the game skews very heavily in favor of multi-classing (albeit less so if you are a mystic). Specifically, it behooves almost everyone to pick up a level of rogue somewhere - because you get 3 bonus tags, and that is HUGE. Even if you never use the rogue class ability, the fact that you grabbed three tags for that one level of rogue is pretty great. You diversify a lot with that one level of dedication. There's not much benefit to being completely committed to your one class - even for a rogue, taking a level of mystic or warrior is going to give you a bit of extra flexibility.
But is that a design flaw, or is that a strength of the design? You're going to mix and match within those three options (usually taking at least two) to build your character. As an adventurer, what are the odds you don't learn at least a little bit of magic? What are the odds you don't at least pick up the rudiments of picking a lock or disabling a trap? What are the odds you don't at least try on some heavy armor and learn the basics of wearing it?
Because I like the idea that everyone is just an 'adventurer' with a different set of skills and abilities, but all basically mutts. How many 'pure' class archetypes do we see in fantasy literature? I go back to LOTR - the dwarves in the Hobbit all had smidges of rogue to them, and Gandalf did go around with a two-handed sword. The four hobbits of the fellowship are generally rogues, but they picked up a smattering of magic and warcraft along the way. Samwise Gamgee is a gardener who becomes a member of the fellowship. Giving him a smidge of wild magic is completely on brand.
That said, I feel like there should be some reward for dedication to a particular class line. I like the idea that at some level (maybe level 4?) there's a special ability that kicks in. If you are dedicated enough to get to level 4 as a rogue, or a warrior, or a mystic of a particular type of magic, you should get some value out of that, too. I'm not sure what that looks like yet, but it's a layer I'd like to add.