The seconds display is used extensively in the configuration of the clock, and it becomes a bit difficult to know where you are if you cant see the seconds digits.
There's also the problem that the HV generator is controlled currently to only work when the digit is displaying (because in that case the HV is loaded). If we don't load the HV generator for the seconds digits, you'll find that the HV rises during the seconds display and this will cause slightly uneven brightness on the hours digits.
Having said that: If you use a WiFi adapter you can get around the problem with the confguration, and it's a simple code change to only display 4 digits, not 6.
This question comes up again and again, so I am starting to feel that a 4 digit clock firmware is a sensible extension to the range.