The magus is already giving up one quarter of their time to train an apprentice. Making them work the other three seasons a year probably makes this a wash. Here's a discussion from a few years back on what sort of work an apprentice can do for you:
I don't think that is advantageous to the order as a whole to have apprentices with strong scores in the arts rather than have apprentices that accomplish work for their mentors. I think it far more likely that the goals of the mentors are more laudable than the goals of the children adolescents and young adults that typify apprentices, considering that, in addition to their youthful predilections, they also have less experience and less understanding of their world.
In character words from a grumpy old magus "The life of a magus is filled with work for oneself instead of for others. To push this selfish behavior back into apprenticeship isn't a noble goal.