I'm not sure why people are so opposed to this. This is a good idea in my opinion. It makes a lot of sense to need at least one hand free to use magic. It's not making battlemages useless at all; you just have to use a one-handed weapon to cast spells.
But without a shield, are they really ready to fight at melee range? Battlemages don't usually have a warriors health.
yeah the speed thing does seem better. it makes sense since you not as limber in heavy plate armor as you would be in leather armor.
magic may not appear until you cast it, but it should have a semblance of some kind of concentration. its just silly to spam fireballs in the middle of combat while you swinging and blocking. it makes magic essentially just a gun and doesnt have any downside. if i pick sneak and lockpick as majors i have just neutered myself because i could have picked illusion and alteration and done the exact same thing and had extra spells as well. i mage shouldnt be capable of everything..........where are are the downsides and the reasons NOT to pick magic skills.
You can't swing while casting in Oblivion. You can swing soon after though. Also, magic use for characters that don't specialize in magic is never as effective as a full mage's spells. The spells that can be cast are usually weaker, and there is rarely enough magic to cast more than a few spells in one fight, and that little bit of magic regenerates slowly. Not to mention that the cost of picking a birthsign with a large magic bonus has terrible penalties for melee characters. Weakness to magic is a big deal when spells pass through armor.
And sneak and lockpick give different attribute bonuses than illusion and alteration. The agility increase makes marksman more powerful, as well as making the other skills governed by agility more effective. It pays off for stealthy characters.
The downside to picking magic skills for characters not based in magic is the loss of attribute modifiers and the difficulty of balancing magic to melee/ranged use while leveling. Too much magic use and they are an underpowered mage. Too little magic use and they have wasted major skill slots, which leaves them with few useful skills.
And pure mages are squishy. They have little health and, without armor, little reliable defense. Shield spells are only so effective when you have 100 health.