Hi @Silverfox, one example that might be interesting to financial industry sites is the way that a network adversary can rewrite links on the page so that even if the payment pages are HTTPS, a user would be tricked into going to a non-HTTPS version (or a fake site).
A comparatively low-effort version of this is automated SSL stripping, described by Moxie Marlinspike.
http://www.thoughtcrime.org/software/sslstrip/
So, even if part of the site is "supposed to" be HTTPS-only, if a user starts interacting with the page in HTTP, a network attacker can rewrite all of the links so that they're still apparently HTTP, and then proxy the requests so that the HTTP redirects that the site sends are hidden from the user. This attack is extremely effective, and is part of the motivation for HSTS.
People might think that this sort of attack is rare, and I agree that phishing is a more common vector for financial fraud today than network attacks. But it's very easy to go to some public place and create a "Free Public Wifi Network" that other people will use, and it's also quite possible to compromise a lot of public wifi routers. So I think it's credible that the prevalence of these attacks will increase over time, especially if sites fail to adopt HTTPS.