Mastering a class already gives you these bonuses:
Priest -20% magic damage taken
Thief +100 attackpoints
Warrior +1 to damage multiplier
Mage +20% magic damage done
Hunter +100 defencepoints
Ranger -20% melee damage taken
Gypsy +20% melee damage done
Esper +25% magic damage done
which are pretty obviously powerful just on their own.
Someone with 20 remorts say, does indeed potentially benefit under this, in that they will notice more skills/spells available to them. They still do not have all the masteries a GM currently would have. The people that have worked for the bonus of GM still have all the masteries they achieved, and they also will find that instead of having all of the tier 2 skills and spells, they will have up to the tier 4 skills and spells of their last class as well.
Having the skills and spells of another tier 4 class is arguably better than having all the tier 2 skills and spells, but the main benefit is that it allows us more variety and dynamics within the classes than just: Hi, I'm an everythingclass.