13pt font at 72dpi is actually a pretty decent body font size, and helvetica renders not altogether horribly with these settings, so it's a reasonable choice for designers testing only on macs.

That said, yes world, please don't use Helvetica for body font, most people don't even have a good version of the font, and plenty of renderers butcher its kerning, anti-aliasing (especially with versions of the font without good hints, or with renderers that ignore them), and even its baseline.