This article by noted graphic artist Mark Simonson, who specializes in lettering, typography and identity design, explains why Arial font-face is everywhere and why that’s not a good thing. The history of the use of fonts in digital media is important study for all, not just designers. (We even get an early view into Microsoft’s character).
The few cases that I have heard of where a designer has intentionally used Arial were because the client insisted on it. Why? The client wanted to be able to produce materials in-house that matched their corporate look and they already had Arial, because it’s included with Windows. True to its heritage, Arial gets chosen because it’s cheap, not because it’s a great typeface.