Hmm that could work for an unsubscribe page, since the user is motivated to make the second click. However, for an email with a sign up link or a subscription confirmation link having an extra step could reduce your conversion rate somewhat.
but OTOH semantics of GET allow e-mail servers to implement a link scanner that e.g. checks all linked pages for phishing.
That breaks (and likely is broken already in practice) when pages don't respect semantics of GET and unsubscribe/verify, etc. when page is merely displayed.
