Just wondering if there's any advantage of going no-armored instead of heavy-armored for a Spell Sword type of character, assuming I will be crafting and enchanting my own heavy armor.... ?

I am under the impression that both light armor and heavy armor can hit the physical resistant cap. I would assume that heavy armor with smiting, can reach the level with less perk investment in the first perk. I do believe that, with alteration, you do not quite hit the cap.

So, looks, mobility and other factors come into play

But with the steed stone or the related perk in heavy armor makes it as mobile as non-armored. So I guess that with enchanting heavy armor will be much more viable and powerful than robes..

Advantages for going Robes.... say as a true Mage... well, you are a bit more squishy, but you do get a LOT of carrying capacity space in Stamina than armored.
While your armor rating will be very low, you can spec into getting Mage Armor to increase spells like Oakflesh strength, and you can get added bonuses for not wearing any armor at all. You will have to pop the protection spell quite a bit though, and it does not really thwart a heavy damage hitter, will have to heal and use potions more. Can go with a combination of say Robes/light armor gauntlets, light armor boots to give you a bit more in armor rating but it still will not be enough compared to Light or Heavy armors.
Light Armor is a good fit for a Battle Mage type of character, as you are not in all Heavy armor which slows you down, and can also get bonuses for wearing all Light armor or matching sets of armor. Go do the Thieves Guild or Dark Brotherhood quests and you can get some awesome Robes or Light Armor for that type of character without worrying about crafting it.

But doesn't the Steed Stone or the "Conditioning" perk in Heavy Armor completely eliminate the extra stamina requirement for heavy armor, again making it better than robes in all areas??

For a pure mage who doesn't melee it is better to use robes. For a spellsword who wants to fight up close it would make life difficult. You could try just using a shield and grinding block up at the start of the game. You would really need to concentrate while fighting though.

Even for a non-melee mage, WHY would robes be better? If I can dual-enchant a set of Daedric or Dragonbone armor, why would I wear robes? (assume I'm using the Steed Stone or have the Conditioning perk)

elven armor for any class. the only advantage of robes is role playing, the only advantage of heavy armor is looks.

Ok Lendial, light armor does have a disadvantage of requiring smithing if you would like to have survivability

Full set of Elven has an armor value of 64 excluding shield. With additional 25% from Matching Set, and 25% for Matching set, it will give you 96. You need all 5 perks in Light armor to get you to 192. I don't know how the invisible armor bonus works or works all the time. Cap is 567 You will need smiting or lord stone plus alteration casting (which can also be tedious to others).

So if a class doesn't utilize smithing, than it could bite you back

Edit: Due to armor change with skill, you need to reach a base armor value of 85. All info can be found at http://www.uesp.net/wiki/Skyrim:Armour

Because as a mage you should almost never get hit by a weapon. You get the disadvantages of armor but don't have a use for it. If you don't grind or train it up, it won't increase much catching one or two arrows per dungeon. If you do train it up and put perks into it to actually make it effective, that gets you to the soft level cap faster and makes you less effective at magic.

It's all personal preference.

I use robes on a pure mage and light armor on anything else.

i guess i shouldnt assume everyone pursued smithing. regardless, i dont think the defense value from full deadric is much higher than full elven once you factor in the extra perks required to smith the former.

translated into damage reduction % the difference would be even smaller.