Would you like to see Skyrim humanoids have birthsigns and use their birthsign powers against the player and each other? Would you like to see birthsigns and associated powers receive makeovers for them to be useful to everyone?
Powers that enemies could have used in Oblivion:
The Apprentice
Lots more magicka, permanent
Absolute weakness to magic
Because of the above, spells such as fortify magicka 100 is amplified on the Apprentice
The Lord
Blood of the North - Restore health several times per day
The Lover
Lover's Kiss - Paralyze a close target at a cost of fatigue, once per day
The Ritual
Blessed Word - Turn undead to your bidding multiple times per day
Mara's Gift greater - Restore full health, once per day
The Serpent
Serpent Spell - One spell with four effects, once per day
* Strong poison on close target
* Cure self of poison
* Strong dispel magicka on self
* Strong fatigue penalty to pay for the spell
Cobra's Dance - Acquired from Serpent Birthsign Doomstone
* Stronger poison and paralyze on close target, once per day
The Tower
Tower Key - Open average lock to get at a locked enemy, once per day
Tower Warden - Reflect damage to attacker, once per day
Warden Key - Acquired from Tower Birthsign Doomstone; Open a hard Lock to get at a locked enemy, once per day
The Master's Hand - Strong boost to armorer for a short time, once per day
The rest of the birthsigns had passive effects, so I did not list them.