On Sat, 28 Dec 1996, Markus G. Kuhn wrote:
8) You could make the minute part of the time zone optional and require that time zones with an integral number of hours offset to UTC (which are almost all!) should only be represented by a 2-digit offset. That is what GNU RCS does already and I think having only a single colon increases human readability.
I've got a better argument for why this is a bad idea. Many people implement by example and only look at the standard when things break. If we make the minutes optional for the offset, these people will probably never see minutes and their implementations will break in those countries which use minute offsets. So in the interest of interoperability, we shouldn't make the minute part of the offset optional.