Well, as far as I know Imbuing isn't to Mysticism as Eval is to Magery. Imbuing is to Mysticism as Inscription is to Magery, in a support skill (additional damage) kind of way. In the same light, Inscription doesn't game from casting Magery skills.
I say that because Eval Int also modifies some Mysticism spells.
...just my reasoning...
So... it's sort of like this...
Eval is to Magery as Eval is to Mysticism.
Imbuing is to Mysticism as Glass Blowing is to Alchemy...
Which is to say, you don't "need" it, but if you're ever going to use some of the spells it requires, you should probably have it. Me, I've resigned myself to the fate that my Rising Colossus is going to be bleh... my animated weapon is going to suck (I'm DEFINITELY not putting tactics and a weapon skill on my mystic mage). And my spell trigger just won't ever really be used, much like my healing stone.
That still leaves me 12 useful spells, so I guess I'm okay with that. Makes me better off than Bushido still... *snicker*