If you have ever visited Scotland, you are probably well acquainted with Scottish dialects or at least with what you have been able to decode from the torrents of words you are encountered with. Even though I am not an expert, I can rightly claim to be able to understand some Scottish dialects after having lived a year in Scotland. Well, only until I meet the odd Dundonian or Glaswegian that shatters my illusions. Sadly, these occasional encounters have become the rule.
Having spent a few days in Scotland last week, I was shocked and amazed at how fast one can lose the ability to understand dialects. Getting one’s ears attuned to a specific dialect or accent is a hard piece of work. Not only being able to understand a dialect but speaking it, is even more difficult. In this BBC article, with the lovely title In yer ain wirds – What might we lose if we all began to speak like each other?, the question is raised concerning the loss of dialects due to favouring more standard varieties. Only recently the last speaker of the Cromarty fisherfolk dialect has died. (You can listen to a short recording of this dialect here.)
Technology, but also ideologies have taken their toll on the great variety of English dialects. Being forced to speak a more standard variety in formal contexts or at school is something I can personally relate to. For me, my dialect is my mother tongue. One has to bear in mind that language is always more than just words. It is about identity and culture. Why then prescribe a certain accent or imply that one dialect is inferior to another one? Sounds familiar?,a project of the British Library, allows you to eavesdrop on speakers of different British dialects and accents. Have a go and if you do not understand everything remember to haud yer weesht an’ get oan wae it!
P.S. Just to be clear about the distinction between accents and dialects: Accents concern pronunciation, whereas dialects involve even other features of language such as vocabulary.
Bridging the Unbridgeable 
I think you’ve almost answered your own question here. Since language is used to define and communicate identity and culture, one group — particularly a culturally dominant — may prescribe its norms in an effort to reinforce its identity within the wider community. That identity will often be carried by the dialect that is called the standard language, which is used to assert the dominant groups cultural primacy. This is at least a good model for the Anglophone situation, I think.
Other languages work differently, especially in cases where there is not strong acceptance of the standard language ideology. In Norway, for instance, there is no standard in the spoken language — although they do have as much as two written standards! (called Bokmal & Nynorsk). So according to Ernst Hakon Jahr, all Norwegians speak dialect. I suspect, though, that there is still a social stratification of these dialects, where some will be considered more prestigious than others.