Apart from the 'professional dress can display a professional attitude' debate (to which I subscribe but which which will never be resolved on this forum), one of the arguments I often hear for shorts and flip flops is that it is a hot day outside. In this unit at least (Swanwick), the temperature is a constant 21-23 degC 24/7, 365 days a year. The same individuals who use the weather as an argument for a certain casual attire in the summer strangely don't sit in snow boots, hat, scarf and gloves in the Ops Room when it is below freezing outside.
It would seem that the argument falls over at this point... perhaps it is more about wanting to cock a snook at management?