Ok, well the bug was maybe fixed, I could learn Shapeshifting without Morrigan.
And as others have mentioned, once any spec is unlocked, you don't need to 'learn' it again. That's why tomes for unlocking them aren't available for subsequent toons if you already learned the specialization during an earlier playthrough.
I think a companion's 'can you teach...' conversation option remains available for unlocked specs because it would be a coding headache to remove it.
Learning a specialization can also be a roleplay device... for example, my Cousland Warden, Moira, is a S+S Warrior who I like to play as a Champion/Templar... much like the traditional build for Alistair, actually. I could give her Templar @ level 7, even though Alistair doesn't ever like her enough at that point to 'teach' her... but I don't - despite the spec being available and even though it would really be handy for annulling the Circle.
Rather, I prefer the roleplay of establishing a friendly relationship between Alistair and Moira and his finally acquiescing to instruct her in all things Templar as the foundation of their romance.
(Because nothing says 'romance' more than two people dressed like this: 
