An example with a four-school limit: if you have points in pyro, aero, necro, and warcraft, you cannot put any points into hydro or two-handed unless you abandon one of the other schools (remove all of your points from it, probably at some cost). This allows you some flexibility to hybridize and change over time without being able to put one point into every school for the utility skills.
Is this a perfect solution? Certainly not. But in design it's pretty common to come up with good solutions by first looking at extreme examples and edge cases and finding useful components of those.
