Even before people get to the aesthetic qualities of Arial and its non-free licensing, there is a practical problem with it: It is nowhere near being a "global typeface", as this page suggests, because it covers very few writing systems: Latin, Cyrillic, Greek, Hebrew, Arabic and maybe Thai. It doesn't even cover Latin, Cyrillic and Greek well and has a lot missing characters.
Forcing Arial on languages that it doesn't support is pointless: they will change it in local CSS anyway. This includes languages of West Africa, which are written in Latin, but use many characters that Arial doesn't have.
I don't quite understand why should we specify an explicit font-family in the first place. People who read in languages for which fonts are easily available should be able to use their own preferred font (unless Wikipedia wants to create a unique identity for itself, but Arial is obviously not a font for unique identity). For people who read languages that Arial doesn't support at all it can a be a problem - millions of people still use browsers that don't support fallback fonts, and they will be forced to see squares instead of letters.