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.
I also find weird the choice of Arial for Latin languages. The primary font should be a free one and then Arial can be suggested as fallback for browser compatibility. See w:Arial#Free_alternatives.
See https://lists.wikimedia.org/mailman/listinfo/design if you'd like to give this a little more formal exposure (which I wholeheartedly support).
I thought I'd seen this discussed on a mailing list before, but it might have been "internal e-mail". Anyway, I search the archives of design and only found this post on fonts so far:
Other recent news are the release of Adobe open source fonts Source Sans Pro and Source Code pro. While the industry trend is to go for a small common set of open source fonts motivated by cross-desktop and cross-browser compatibility, we can't have precisely Wikimedia going in a opposite, old school direction going for... Arial.
I will be having a presentation in 4 weeks at FOSDEM and I want to take our design guidelines as inspiration for that slide deck. I don't know what fonts will be used but I can assure you Arial or any proprietary font won't be there. Any suggestions?