Intelligibility ... is one of the two big forces that are driving language. The other big force... is the need for identity: I have to say who I am and where I am from. And that produces local accents and dialects, both nationally and internationally. So it’s the tension now between these two forces, intelligibility saying let's all speak the same, and identity saying no, let’s be different. (starting at around 3:10)
This, by the way, can also be heard, almost word for word, in another interview:
Or, in a slightly more nuanced way, from the standpoint of listener rather than speaker, in his book on English pronunciation, Sounds Appealing:
My unease with this position is the express intentionality of signalling one’s identity. Of course our first acquired language (a dialect of a language) will likely leave a more or less permanent mark on the way we speak. This mark, this identifiable pattern of characteristics, usually colors our speech quite independently of our volition. In Crystal’s phrasing, however, it seems that the speaker expressly wants (or perhaps even should want) to sound like he belongs to a particular group of speakers. Yes, undeniably, this happens, when someone really wants to belong to a certain group or to let others know that he belongs to a certain group; but my question is, how often does it happen? Is it really a big driving force, that someone from Manchester, Liverpool or Newcastle consciously wants to sound like they are from Manchester, Liverpool or Newcastle, etc.?
As a side note, Crystal says he likes to hear English spoken with a foreign accent:
...it is the identity drive that I am fascinated by. I love to hear English spoken with a Spanish accent... I love to hear English spoken with a French accent — why? not just because it’s delightful, but because it helps me identity who the speaker is. (From the first video above; I’ve also heard him say that somewhere else, but can’t find where.)
That’s a phenomenon, probably within the purview of sociolinguistics, that I’ve long wanted to express my amusement about; because for me, while almost all native variations of English I’ve heard so far are fascinating, none of the foreign accents in English that I can recognize is in any way delightful.