dtgreene: It depends on which game in the series you're talking about.

For example:
* In Daggerfall (and, to a lesser extent, Arena), offensive magic can be very powerful, but you don't have enough Spell Points/Magicka to use them all the time. Therefore, you're going to need some weapon to use when you don't want to use your magic. (I believe that, in Daggerfall, you could do fine with just your bare hands. I know that's actually a very good choice in Battlespire, because weapons have drawbacks that don't apply to unarmed combat for whatever reason.)
* In Morrowind, Magicka still is limited (no recharging without rest until Oblivion), plus the majority of stronger enemies have 20% spell reflection.
* In Skyrim, the charge cost and effectiveness of Destruction enchantments on weapons are affected by your Destruction perks.

Also, worth noting that, except in Oblivion, Heavy Armor doesn't affect your spell damage at all. (It's worth noting, however, that in Arena there's no class that can both cast spells and wear plate armor.)

thegregorsamsa: ... I just realized that there is not any Oblivion-specific forum :(

Greywolf1: This is why (most) people mention the game name in the thread title.