Remembering both vermouth and gin are flavoured with similar botanicals, they obviously have an affinity for each other. This drink may be simple but made...
I've just been rereading David Embury's 1948 classic, 'The Fine Art of Mixing Drinks', and it suggests the great man was no fan of the Gin & It. He says: "...there is always likely to be something wrong, something not quite satisfactory, about a blend either of dry vermouth and whisky or of sweet vermouth and gin."