It might be that some treatment approaches are better able to facilitate good therapeutic alliance than others.

I agree that there are limits. From memory... People with borderline personalitly disorder also have a disposition to join cult groups and churches and movements and the like, things that promise a relatively 'quick fix' by a charismatic leader. I'm sure that a charasmatic faith healer or channeler or spirit reader or whatever would offer a relatively 'quick fix' of acceptance in the first place but that... It would wear thin over time and there would be high drop out rates (which is of course a significant problem with people with BPD under any mode of therapy).

I think that internal consistency and consistency with scientific practice is nice. That being said I'll admit that I have difficulty with seeing a unified theory behind the practice of CBT and I think of CBT as an eclectic tool kit of strategies that have been individually empirically supported but I'm not at all sure there is a coherant or consistent theory lurking behind the scenes... The 'cognitive' in 'cognitive behaviour therapy' also doesn't seem to be (in most cases) inspired by (or informed by) the most recent research in cognitive psychology and I think it is unfortunate (and misleading) that they share the same name.

> Does anyone really believe that all the different therapies that have been developed are equally valid?

With respect to the ones that have been empirically supported... E.g., brief psychodynamic, schema therapy, DBT, humanistic etc... It might well be that they are ALL affective because they effect change on the SAME mechanisms. There is a certain amount of translation between theories that is possible and while different theorists might call the constructs by different names they can be the same thing really.

E.g., what is the difference between learning how to mindfully 'observe and describe' ones mental states and...

learning how to 'mentalize'?

Sounds like the same thing to me (but the first is in the language of CBT and the second is in psychodynamic terminology)

(Mentalization also has links to the notion of 'theory of mind' (TOM) as discussed in cognitive psychology, or the ability to 'adopt the intentional stance' as discussed in philosophy).

CBT has been shown to be effective for the treatment of BPD?

Reference please...

I know about schema therapy (cognitive therapy)
I know about DBT (dialectical behaviour therapy)

Are you using CBT to refer to the above two or is there something I'm missing?