In both Morrowind and Oblivion, it pretty much was not viable to play a pure Mage. The problems were very widely acknowledged by the player community, and there were lots of mods created to make pure mages at least viable (although some, like Mighty Magick, went overboard). I played Skyrim at PAX in Seattle a couple of weeks ago, and it still seemed that Magic was a bit on the weak side -- although, it was hard to tell based on 15 minutes of gameplay.
Morrowind had extremely powerful enchanting; but outside of that, both it and Oblivion suffered from the following:
1. The size of the magicka pool was insufficient to cast enough spells to make it through combat.
A melee charater's fatigue was both large enough and it regenerated fast enough to easily handle any combat situation. Melee rarely had to consume restore fatigue potions. For mages, the size of the magicka pool was tiny. Morrowind didn't have combat regen of magicka, and Oblivion's regen rate was too slow.
2. At starting skill levels, the problems for mages were far worse. It wasn't until spell skills were in the 90s or at 100 that magicka cost was within a range that could be considered to be close to viable. At starting skill levels damage per magicka was over 5 times what it was at skill level 100, making it literally impossible to play a pure mage -- you were forced to use melee skills, and they would make up the bulk of the damage that you did.
3. The durations of buffs were extremely short-lived, resulting in the need to have to extremely frequently re-buff.
4. Because of both the short duration of buffs and the heavy magicka costs, it wasn't practical to use spells for survivability -- most players wound up wearing armor.
5. Melee characters had to focus on only a few skills to be great: block, heavy armor, and a weapon skill. Mages had to focus on many more skills, and even then wound up being sub-par to melee types.
6. Because of the skill glut that Mages faced, plus the fact that you would run out of magicka quickly, it was hard to get spell skills up to decent levels in the 'level race'. This resulted in the correct levelling strategy being not levelling, until spell skill levels had reached decent levels.
For both Morrowind and Oblivion, melee characters wound up being both more survivable and more damaging than Mages: superior in all ways.
I'm really looking forward to Skyrim -- I've played all of TES since Arena -- and I have my Skyrim CE already pre-ordered. I really hope that Bethesda has finally made Mages fun to play in Skyrim without the need for mods.