[jakeludington]

Many email services use an implied open which is based on clicking on a link in the email, because you can't click on something you never opened.

Those people are actually more active than someone simply reading an email and if you have an email list full of subscribers who never click on anything, you have a very disengaged list.

[JoshTriplett]

Depends heavily on the email in question. If your email exists primarily to point people off to links elsewhere, then sure, you can measure "success" by how many people click those links; if you actually put useful content in your email, people might read your mail for its content and get value out of it whether or not they click the links.