Microsoft did not want to pay font license fees for Helvetica and designed its own Arial knock off. Arial definitely does not look as good as the original (earlier post here). Why? There are only minor differences in the characters.
I think the main reason is the availability of weights. In Helvetica, I like using the light and medium font weights. Arial installed on my machine comes in a blunt regular (somewhere in between light and medium), and has a bold that is too heavy.
What do you think?