Mysticism is one of those secondary skills, that have a good idea but the execution fails. A hero with Expert Mysticism gains four spell points per day, which is the same amount of points a hero with Mystic Orb of Mana (a treasure class artifact) would gain. The only situation mysticism could be useful, is in maps with long traveling distances or maps with no magic wells or mage guilds to recharge spell points. However, these are rare occasions, of which the Dragon Slayer campaign might be one. In conclusion, mysticism is just not worth spending one of eight possible secondary skill slots.
Assuming that numbers are rounded down, it takes speciality Mysticism a while to be useful as well. At basic level, you will restore 1 additional MP per day for every 10 hero levels (10 * 5% * 2 MP), at advanced level, you will restore 1 additional MP per day for every 6-7 hero levels (6.66 * 5% * 3 MP), and at expert level, you will restore 1 additional MP per day for every 5 hero levels (5 * 5% * 4 MP). Surely by the time you reach this level, you should at least have access to one of the pendants of mana or similar artifact?