I don't even know where to start with this... Mana evaporation exists because the first director behind mabinogi hated magic and weaved a lore reason into why it's stupid. I'll hand it to him, it's a neat way to get your hate across. Further that with the long RNG based drop quests and high repair costs with decent rates and the fact that meditation, at any rank, is literally worthless, and wooohboy, what a good set up.
"Preventing you from easily hybridizing" you mean like...literally every other skill set? Holyshit, I guess switching from a bow to sword better nef my dex by 75% for 5-10 minutes! That'll sure make me think twice about doing two styles of play I enjoy!
Never, EVER, use that chain cast excuse. That is, no mater what, a BUG that should be fixed. Everyone can say "Oh, mana evap is a lore reason!" So what's the lore behind chain cast, that would normally cost mp for each bolt loaded, suddenly NOT NEEDING the mp if I don't have enough...? Not one? Oh, well, that sucks. I will never see this as anything other than a bug that was never bothered to be fixed, as it makes no sense lore or game-play wise.
Also yeah, manashield isn't a mage skill. That's a joke. That's a skill for everyone who doesn't use their mana constantly. As a pure mage, it's sorta trash. As a hybrid, it's sorta nice, as ANYONE ELSE that doesn't even touch magic, it's glorious...which is weird but I guess okay.
Sadly the bonus is really minor outside of upgrades, and even then only chain cast for bolts, which is even more of a reason that mana evap is crap... Meanwhile, aside from cylinders, as their skills boost in damage quickly, basic bolts in the early game deal 'eeeh' damage, while even a rank N combat and sword mastery person can dish out 50+ damage with upgraded swords per hit quickly. That takes time, but as repair costs for melee and ranged weapons are much smaller than wands, it's something they'll get from just playing normally instead of trying to save up for it in the early game.
I'm not saying that chain cast mage isn't the only option. Anyone can play how they want, and to be blunt, the int magics do not hybrid well honestly. Thunder locks you to the wand, IS, FB, and TH all cause displacement lag if you dare move while they explode/strike. It's just a little silly that a bow or cylinder can be freely swapped out for a melee weapon (or each other) with no downside what so ever, but if you wana take 30(25?) seconds to launch a fireball and swap to a bow or some swords or a cylinder or just your fist or a different wand to take out stranglers (which there will probably be plenty/a few), you get punched in the face with a massive penalty.