All things considered and with perfect hindsight, did the Allies benefit, by having Italy fight on Germany's side rather than remain neutral?
I was inspired to ask this question after reading Paul Kennedy's rather amazing claims in The Rise and Fall of the Great Powers (1988). On p. 298:
In 1939 and 1940, the western Allies frequently considered the pros and cons of having Italy fighting on Germany's side rather than remaining neutral. On the whole, the British chiefs of staff preferred Italy to be kept out of the war, so as to preserve peace in the Mediterranean and Near East; but there were powerful counterarguments, which seem in retrospect to have been correct. Rarely in the history of human conflict has it been argued that the entry of an additional foe would hurt one's enemy more than oneself; but Mussolini's Italy was, in that way at least, unique.
And also on p. 340:
Had Italy also joined in the conflict in September 1939, its own economic deficiencies might have posed an excessive strain upon the Reich's slender stocks and, arguably, dislocated the chances for the German westward strike in 1940. To be sure, Italy's participation would have complicated the Anglo-French position in the Mediterranean, but not perhaps by much, and Rome's neutrality made it a useful conduit for German trade—which is why many of the planners in Berlin hoped that Mussolini would remain on the sidelines.