They're still useful in the right circumstances, without making low level Mystics ridiculously more powerful.
Take someone with a base accumulation of 10, three levels of Accumulation Multiples, Intelligence 10, and Power Sacrifice.
Under the standard rules, they would have a normal Accumulation of 40, with the possibility of using the daily maintainance maximum of 10 zeon to raise their Accumulation to 70 by passively spending 60 zeon whenever they want. Any spells cast fast would cost up to 60 zeon more, but that is passive, rather than accumulated zeon.
Taking the advantage before applying the multiples, and the Mystic would jump to an Accumulation of 60 for 10 passive zeon, and could boost as high as 160 zeon in a round. What Mystic would not want an ability that quadruples their Accumulation at low levels, and would still be a huge boost at higher levels?
Now give both Mystics Elemental Compatibility, and they run 60-90 MA for the first, and 120-240 MA for the second.
Use a +5 MA Amplifier. The first goes up to 45-75 MA, while the second goes from 60-180 MA.
Combine the two, and you get 65-95 MA for the first, and 140-260 MA for the second.
That's a lot more power than a low level mystic should be able to handle. Compare the values to the Innate Magic table if you want a further example of the power boost.
Power Sacrifice lets a mystic get a spell off faster, but with a cost. It isn't supposed to turn a first level Mystic into someone that could sling 200+ zeon in a single round.
Any time you are working with multiples, they need to be applied carefully, especially if you plan on using them after other things can add to the base value.