Well I must say I found nothing wrong with the way it was in Morrowind, but after having played Oblivion for the last one and a half years I don't know if I could get back to it that easily. It's just too convenient.
But in Skyrim they have changed it to a system where you can equip spells just the same as weapons for each hand separately, which was not possible in Morrowind. So you might not be able to carry a weapon and a shield and cast a spell, but you can have a weapon equipped in the one hand and a spell in other, so it's still different from both Morrowind and Oblivion. In Morrowind it was always only Weapon/Shield or Magic, in Oblivion you can have all at once and in Skyrim mix Shield/Weapon/Spells for both hands individually. Though I admit I still would prefer to be able to cast spells while using a two-handed weapon.
Basically, spells work like items in Skyrim, so I could imagine that swift casting like in Oblivion could maybe only be realized with some kind of autocast hotkey (like Enhanced Hotkeys in Oblivion), but it should automatically equip whatever shield or weapon was used before after the "cast" button is released. Don't know if such a mod exists or is even possible.