Magic Theory* (p.63 and p.66)
Note the asterisk.
If a character has no score in an Ability, she may or may not be able to use it... If it is asterisked, she cannot use it at all.
So it can't be used. But does a lab total use it?
LAB TOTAL: Technique + Form + Intelligence + Magic Theory + Aura Modifier (p.94 and more spots)
Yup, you use it in the lab total, but you can't.
So you need some experience in MT to allow using it to allow for a Lab Total. That makes it a requirement for Hermetic lab work.
.
And, separately, there are things like you can't use vis in the lab without a score in it, so it is a requirement for a lot of lab work that way as well, even if it weren't required for some things:
The amount of raw vis that a magus may use in a single season is limited to twice his Magic Theory score. (p.94)
Charged items are even debatable despite not using vis. Does this fall under the "compound item" rule, where you would be limited to no components? "Compound item" isn't truly defined except to allow you to use all of the components. All of one is one. But that doesn't feel very "compound." That's why I say it's debatable.
A compound item of this sort may not contain more components than your Magic Theory score (p.97)