Base-10 Gematria

A chaveir on the Avodah mailing list (24:1:7) posted a vort regarding the first two words of this week's parsha, noting that the term "Lech Lecha" is well-suited to the theme of Avraham's tests, being that each word has a numerical value of 50, so that the sum is 100, symbolizing that Avraham passed his tests with a score of 100%. I'm not really a fan of gematria-vorts (unless the discoverer is someone on the madreiga of R' Yaakov ben HaRosh) but to each his own.

Another chaveir (24:2:10) challenged the vort, noting that the decimal system is an arbitrary convention, and that in Judaism, 60 is used to represent completion rather than powers of 10.

I responded to the objection, noting that gematria itself utilizes a base-10 system in which powers of 10 represent levels of completion, i.e., (10^0)x, (10^1)x, and (10^2)x. For argument's sake, one could as easily conceive a gematria system using a base of 7 - another "whole" number in Judaism - in which the values are 1,2...7,14,21...49, 98, etc.

Having written this, I began to wonder: why is gematria apparently unique within Judaism in its basis on the number 10?

Ideas?

Update, 10/16: Other posters noted that the entire numerical system of the Torah assumes base-10, weakening my question regarding gematria, but I still believe that 10 - and its powers - show precedent in referring to levels of completion. Dib'ra Torah k'lashon B'nei Adam, after all.