I thought it was completely arbitrary, as well as totally unnecessary. I beat the game, five starred every encounter, including Vicengetorix, excluding the Long GUI (ha, autocorrect), without upgrading a single weapon once. It was stupid, pointless busywork, and it felt like it.
Nevertheless, I did finish the game, and as such, I find myself viewing this thread with a question in my head: what is with all the FFXII love? That was a game so boring I didn't beat it (got up to about the Suncryst), yet you all seem to hold it in such high regard. What gives?