I accidentally came across this post while searching "McQueen" ...
I think, the simple explanation to Mulberry being unlined is that their bags
are the only bags on the market produced in full-hide leather, full thickness, not a split. All the other designers use the upper layer of the leather only, the lower one is split to use for something else (cheaper items, etc in the industry). By unlining the bag you show off the highest quality leather, as only the best hide can be used unprocessed. Mulberry also do cheaper leathers, and such bags are always lined.
It is the same that in the most expensive fur coats the lining is never sewn on at the bottom hem, so you can inspect the quality of the skin which holds the fur. Demanding lining sounds the same as demanding those coats to have attached lining...
It is a sign of luxury leather, not a cheapo. Unfortunately, Mulberry are moving away from that luxury in the direction of cheaper leathers, lined bags, to satisfy opinions of people who do not know...And also full hide is far more expensive than a normal leather... It is like "why on earth this Bentley has wooden dashboard... how old-fashioned... it should be plastic".
I do not work for Mulberry and am unbiased, but have and know their bags. My favourite ones are unlined, with thickness of leather 2 mm, bag holding the lovely leather smell for years. Such bag is a bentley of leather craft, and does not need lining - you just just the suede eraser if you need to clean it - the bag will last 100 years. not 5 as other brands.