I personally like Finesse, because like every skills, it allows magus to differentiate from one another.
It is possible to have the Craft magus with Puissant, Affinity and Finesse score of 7+.
It is possible to have the spell throwing magus who does not need such specialisation, yet a decent score of +4/+5.
And of course, the magus who has no interest whatsoever regarding Finesse, but instead other skills.
Because it is not an Art, a magus cannot simply decide to become a Finesse specialist by putting a few seasons in. It will take a lot of time and dedication. Whereas if it comes to having the right spell, it is much easier.
And regarding Parma or MR being easy to bypass, honestly, I don't mind. There is nothing more boring from a narrative perpective than passive, efficient protection. It only promote arms & armors race. Where as if there is way to circumvent MR with clever strategy, it gives more interesting story, opportunity to clever trap instead of brute force to pierce a very high MR.
I see the problem that high Finesse gives both an edge in Certamen, combat spell and craft magic. And maybe it is too much and too broad of a skill.
One way to possibly reduce the impact would be that either for combat spell or craft magic you need to take a level of Mastery in Finesse to take full benefit of the skills (somehow similar with the Penetration mastery ability, which enhance its effectiveness).
Maybe the initial Finesse skill is useful in non-combat circumstance, when the magus has time to concentrate and focus his will (making realistic Imaginem illusion, fine control of Rego moving spells with Conc. duration). But for instant, quick application like throwing a boulder, he needs a level of mastery.
Instead of enhancing the Finesse ability, it just allows its use for specific application. I dont' have a full ruling to propose with values and such, but I would be reluctant to remove it.
I would rather see Finesse scope being redifinied that purely eliminated, but of course, that my 2 centimes of € (and removing its use in Certamen would not hurt for example).