So, in Oblivion I play as a mage wearing enchanted Robes and I was wondering besides the speed bonus, do robes have any others advantages over light armot or are they completely useless?

Well, they don't affect your spellcasting ability.

Honestly, its just personal preference.

For example, Kelira will only wear light cuirass and gauntlets, absolutely refuses to have anything to do with greaves or especially boots.

Wearing armor lowers spell effectiveness. A mage therefore wears robes (or clothes) to get all that he/she can from casting spells.

This. From memory, most illusion spells have absolutely no effect if you're wearing any sort of armor.

Mind control spells (like command, demoralize or frenzy from the illusion school and turn undead from the conjuration school) are 'level dependent' and stop at level 25. Fortunately, the game realized many players would level beyond 25 and built in a provision that a level 25 spell will affect any foe regardless of level as long as your spell effectiveness is 100%. If you wear any armor, the best you can ever get for spell effectiveness is 95% so these types of spells will become ineffective vs foes that are near or above level 25. There are several solutions:
-Wear no armor so your effectiveness is 100%.
-Use a small mod like 'Spells be Effective' that simply eliminates armor penalties to spellcasting.
-Stop leveling around 20. With max magnitude spells and 95% spell effectiveness, they will work fine.
-Enchant the effects on a weapon. Your Blade of Demoralize doesn't care if you are wearing armor. Note here that 'calm' cannot be enchanted by the player onto a weapon.
More info here: http://www.uesp.net/wiki/Oblivion:Magical_Effects#Magnitude_to_Level_Conversion

Regarding robes, the do take up both your shirt/blouse and pants/skirt slots, thereby reducing the number of items you wear that can be enchanted.

Highest level of armor give 85% armor rating, but if enchant robes and some stuff with Shield effect we can get 85% of armor rating, we can make Shield spell giving 85% armor

But, to get those stuff above takes a long journey and a lot of things to do, on paper it sound nice, practically it is not

Light Armor give Speed attribute, Heavy Armor give Endurance attribute, Robes give nothing on each level up