Actually a pretty interesting discussion about wrench sizes.

Back in the old days of standard sizes, the fractional measures suggested a hierarchy of popular sizes that pretty much everybody understood - few would choose 15/16 or 31/32 when they could just pick 1".

However, in metric, no millimeter size is really any more obvious than the next. This works great in the 4-10mm range because people need all those sizes, but as you get into the teens and especially the twenties and thirties, you start to need extra wrenches due to nothing more than a lack of standardization.

On an unrelated note, I have a 4, 4.5, and 5mm sockets. I think I even actually needed the 4.5 once or twice (not on bikes).