Greetings Louis, that is a great question. Recently we updated our test to check fonts stacks when using an OS based (Outlook, Entourage, Apple Mail, etc) email client. We added this for a couple of reasons.
1) We want to point out that not all fonts are common in all OS’s. You mentioned Helvetica, which is standard on Mac OS’s but not Windows.
2) We wanted to make sure that if you are using an non-standard font that you are using a font stack, which you are in your example.
If one sets the font to say Helvetica without any other fonts in the stack, it can dramatically affect the way the email will look in OS’s that don’t have that font installed by default.
I hope that answers your question.