Considering Piwik is used by the GCHQ, I find it hilarious.
I don't find it using poorly implemented hashing in the administrative interface to be at all relevant to what they're doing, or why they shouldn't be using it.
Given these known security flaws, it's not a stretch to assume anyone who can see the GCHQ's Piwik server can have that data too, regardless of whether they are authorized.
See below for a small preview of what an attacker could exfiltrate (dissident IPs redacted for a reason):
https://firstlook.org/wp-uploads/sites/1/2014/02/piwik2.png
While we're talking about poor security practices: the privileged username in the screenshot is apparently still the default ("admin"), so I hope the password isn't still "changeMe" ... http://piwik.org/faq/how-to/faq_191/
I can't believe unsalted MD5 is "by design" (https://github.com/piwik/piwik/issues/8753).