If you don't live in the UK, having a fistful of sterling when you get home is not really the ideal situation. It's not far from it going bad, in a sense.
Well yes, but you have to go to a bank and exchange it, and they do their best to charge more - the last time I asked about selling €50, the bank wanted a $5 fee...
Anyway, it seems reasonable to me that people might want to avoid that when they have a perfectly good Mastercard which has always worked in the past.
It really depends on the currency. The odds of Euros, say, going "bad" on a scale of months to years are fairly nonzero. Similar for currencies with high inflation rates.