The Mystic is one of those Occupations where the magic-user's powers stems from a source other than study and application of magical principles; his is the archetype of the innate power-wielder. Not only does the Mystic possess an intuitive comprehension of how to cast spells, he is also a psychic of considerable power. This access to two forms of supernatural power, combined with his inability to gain additional spells or powers any other way, make the Mystic very interesting indeed.
The Mystic likely doesn't maintain a firm distinction between his spell-casting capacity and his psychic capacity, though he likely does make one due to how each form of power looks and feels when he uses them. As the core rulebook itself notes, Mystics are not likely to value formal schooling due to the way that they relate to their powers. Mystics, therefore, favor holistic and non-linear reasoning; strict linear cause-effect schooling is, at best, seen only as something to add to their mental toolbox- at worst, they are outright hostile to the very idea due to their inability or unwillingness to see the merit in it.
Mystics have no infrastructure requirements to their magic-use. Because all of their spells are intuitively discovered and mastered, they have no need to tie themselves down to a specific place; in this respect, they are akin to Ley-Line Walkers. Because they have no strong association with ritual magic, they aren't that obsessed with places of power; they aren't that tied to items of power either, so they have little in common with Shifters or Techno-Wizards (or any other Occupation with similar logistical requirements to their practice). They have no formal ties to supernatural patrons, so they see such magic-users as fettered or enslaved usually (and they would be correct). They are, like Warlocks, a very loose community amongst themselves (and see kinship to varying degrees with others like them) and therefore are often friendly to the traveling lifestyle.
Because Mystic powers are intuitive, they will reflect their spiritual and mental well-being; barring specific mentoring intended to create, in some form of controlled manner, the need to manifest specific sorts of powers (ala Warlocks) the spells and psychic powers that a given Mystic develops are tells to a savvy observer of that Mystic and can give useful insight into how that Mystic thinks and behaves. Whatever mundane skills and knowledge a Mystic has is also indicative of that same internal self, as they will be the ordinary manifestations of their supernatural capacities.
For the Game Master Mystics are very easy to run and incorporate. While not as much of a Gandalf sort that a Ley-Line Walker is, a Mystic can be a more adventurous Galadriel (or, as a NPC, the mature one see in The Lord of the Rings); they work very, very well as seers, diviners, and other sorts whose magic and psychic powers tend towards the seeking and refining of knowledge or the healing of body and mind (and are not nearly so hung up on the mastery of mundane martial skills to defend themselves from attack). It is painless to use this Occupation in your campaign, regardless of where it is, due to the nature of its powers and the personalities that this Occupation typically produces.
Tying it together, the Mystic is one of the most universal of Occupations and can find itself a niche in damn near any RIFTS campaign that's likely to get past the pitch phase. Easy to play, easy to portray, easy to incorporate, easy to handle- easy peasy all around. What's not to love? Roll up a Mystic, just an ordinary Human (or near-Human) Mystic, and take the implications aforementioned to heart when you figure out what your Mystic is and what he does with his powers. Just remember that you're not the blaster-master sort; you're the seer and reader, whose powers allow you to punt problems away by avoiding them or press them down by mitigating them- you're not the guy who casts the spells that makes the people fall down. (That is why you have a gun.)