Mage plate gives the most +mana out of all items if you level high enough. Most folks besides Gnome can't really use it , but she plays a lot of gnomes and levels them up a lot. It's also quite cheap and has ok CP for the cost, so it's kinda worth leaving in the pool regardless. The attack penalty is meaningless, there's a ton of ways to compensate it if you really want to, and a mana boost that size is hard to come by otherwise, and if you're a caster it doesn't even mean that much.
It's one of those things which looks scarier than it is, then when you figure that out it looks better than it is, then when you figure out it's mostly kind of overkill, really. Now that I think about it, it's not that most folks other than Gnomes can't use it, anyone can use it, but are kinda better off just picking it up from a shop if it's there and they want to become an instant spellcaster.