That is a false presumption. The mere fact that you get free mastery in every spell makes it more than worth the balance of a Major Hermetic Virtue. And I am just thinking of published Mastery Abilities.
As for examples, not the best one, but I have this...
Roberto of Flambeau in Novus Mane, 1220 to 1226
A few years later...
Roberto in Bibracte 1228ish
And currently...
Roberto as the old man of the sea
Though I am nominally the ASG of the Andorra saga, I stopped running it and I just play now. The Fixer has kept meticulous track of the development through the years. He recorded some "snapshots" of a few years.
Roberto through the ages
In later versions you will see some house rules Mastery abilities, so I will admit they are a weakness in my argument. As are the HRs for advancement used in my saga as reflected in the latter versions. I was feeling inventive and was trying to incentivize players to study more Mastery It was somewhat popular but nothing overbearing. If I were to redo things, I would cut most of that.
I will say that it is more than obvious that a magus should dedicate the vast majority of their time to learning and inventing spells. But I will also admit that this is a subjective idea based on personal aesthetics. And therefore, I will admit that Flawless Magic is not the only and obvious choice. I say it is gives you the most bang for the buck, but you can only start with one Major Hermetic Virtue and it is most rare to obtain another. What you want to do with this character is more important, and there are certainly traits better suited to your vision.