When I was creating All-That-Is (http://www.all-that-is.net), I toyed with the idea of using a different base than 10, but decided against it, on the basis that for the purpose of roleplaying it would be too tricky for the players to do the conversions on the spot, and honestly, most wouldn't even bother. This is the same reasoning I had behind not giving all of the numbers different names and symbols - if I was writing a background language/numbering system (think Klingon in the Star Trek universe, or the second language in Futurama) then I'd have no problem switching it around, but the whole point was that I wanted there to be a universal numbering system for the whole planet.

Once I'd decided to go with 10 as the base number, I worked on everything being done in base 10. There are 10 races (http://all-that-is.net/index.php?title=Races), which means that there's 10 countries (http://all-that-is.net/index.php?title=Countries). There's 10 orc tribes (http://all-that-is.net/index.php?title=Orcs), 10 Narcarbia Islands (http://all-that-is.net/index.php?title=The_Narcarbia_Islands), and 100 Nevah-Nevah islands (http://all-that-is.net/index.php?title=Nevah-Nevah).

More relevantly, however, the year is split into 10 months (http://all-that-is.net/index.php?title=Months), each of which has 50 days. Each day has 10 hours made up of 100 minutes made up of 100 seconds.

I also really like the number 3, so there are 30 types of travellers (http://all-that-is.net/index.php?title=Travellers), 30 races (if you include halflings and exclude Orgs, which most people do), 3 rogue countries, and 3 rogue orc tribes.