Generally dialects survive in areas which have had little contact with outsiders
Way off! Actually, port cities tend to have the most distinct accents (Liverpool, London docks, Bristol, Glasgow, Newcastle, Hull, Portsmouth, Yarmouth). As someone else said earlier, to natives it's sometimes possible to tell which part of a large city someone is from, just by subtle changes in accent. But the increase in university attendance means it's getting harder - graduates tend to lose/soften/change their accents, just by virtue of being more mobile and mixing closely with different accents.
Re why it is useful to know/be aware of accents - many migrant Poles wont be settling down in the first place they move to, and will tend to be mobile to follow the work, so knowledge of the variation is at least useful. Also pretty useful for academic conferences too!
I don't think a Pole* can ever completely lose their accent when speaking English. I can always hear it, even softly, and for most speakers it's quite pronounced - but all Brits have a regionala ccent of sorts, so it's not a bad thing, it's the same as recognising a Scot or a Brum.
*I have heard Polish-born children speaking English with a flawless native British accent, and also Polish with no English accent, so it seems quite easy for them to switch. But never, ever heard an adult speak without an accent, even those who went to the UK decades ago (as adults).