I'm a little surprised there isn't a greater awareness of sociolinguistic accommodation i.e that individuals tune their accents and vocabulary into different social and geographical contexts as required.
As someone who grew up surrounded by Scots speakers who wouldn't allow me to actually speak the language, I have to say how irritating, patronising and pointless I found that as a child (and not to mention sad to look back and realise how much people didn't value their own mother tongue) From an early age I was more than aware when it was appropriate to speak and write in English and when it was appropriate to speak in Scots (people didn't even attempt to write in Scots back then but thankfully this is changing).
I also find it disheartening to hear the notion of 'speaking properly' still being bandied about. As my English Language lecturer used to say in the early 90s, "the native speaker is always right". There's no such thing as speaking 'correctly' - there's only a generally agreed usage within a particular context.
Languages are always evolving and changing and some innovations stay. For example, the pronunciation of Shakespeare today is totally different to how it would have been spoken at the time. Queen Victoria would also have used post-vocalic r's in words like farm and it was the working class who missed them out (an interesting contrast with the status of RP accents today). Vocabulary, pronunciation and grammar are all constantly developing (split infinitive, anyone?!).
The cognitive advantages of bilingualism and linguistic manipulation in general are also well documented. Learning for yourself how to use language in different contexts is extremely valuable. Children identify with their peers, so a child in a school in a middle class area in a particular location will tune into the linguistic norms of that group and use them.
Being able to retain a natural usage of other versions of language for different contexts is also extremely useful, however. We like people who 'speak like us', it makes us feel that we are on the same wavelength and we naturally mirror the speech of others subconsciously in conversation so it's actually an important social skill.