Just my opinion... I think the Flightsweep rear deck has always been judged rather unfairly. Compared to the 'normal' AMERICAN STYLE Continental Kits of the '50's (which are very 'cool' and 'of the period,' but let's face it, very long, bulky and tacked-on), I guess Exner's answer may have seemed, to un-informed eyes, 'cheap' in it's simplicity.
But there are a number of design precidents for spare tires fared into the rear deck - on EUROPEAN cars! The one that stays in my mind is the Mecedes 540 K Special Roadsters and Cabriolets of the late 1930's up. Those cars had a very similar look to the rear deck (albiet operational). There were SEVERAL European cars which used a similar theme - can't think of them offhand right now - that Exner incorporated into his early '50's show cars, and eventually, the late '50's Imperials.
Is the Flightsweep any more 'eccentric' than the fake 'air intakes' on period Caddy's and Lincolns, or the 'dagmar' bumper guards on other '50's cars? No, it's just less commonly seen, and therefore, often misunderstood. I think the Flightsweep deck was an astute stylistic reference to a European-style trunk mounted spare - without resorting to the 'ungainly' extensions that, while charming, are really quite a 'pain' in practice from what I understand (trunk access? theft? PARKING...? Whew!).