According to the Home Office guidance (August 2014) on the approved English tests, the following applies in respect of your wife's application:
"Cambridge English: Key (also known as Key English Test)"
For tests taken before 6 April 2013: Certificate, Statement of results, Candidate ID number and Candidate's secret number (applicants should provide Statement of Entry if possible)
For tests taken on or after 6 April 2013: no documents required (Scores must be verified using the Cambridge English online system using name, date of birth and passport number)."
END OF QUOTE
As you'll see from the above, UKVI's Entry Clearance Officers/Decision Makers have access to Cambridge exam board's on-line systems to verify KET passes for KET tests taken on or after 6th April 2013 for settlement visa applicants, so they don't need the certificate or Statement of Results.
Having said that, there's no harm in including the Statement of Results in your folder as well - that's what I'll be doing next month, assuming that my lovely wife passes the KET test.