That hardly applies to the normal case of someone who presents with a cold, is told to take a few lemsips or perhaps some paracetemol, and then asks for a cert "cos I don't feel like going to work today".
Can't believe I'm sticking up for doctors against employers, but...
There are key issues to separate out in this debate. There is the long term sick and medical certificates for that and short term.
But we also have to consider employment policies. So for example, some employers allow for a number of days "self-certified" (as in not requiring a medical note). However, some employers insist on medical certificates for even 1 day's absenteeism, therefore people do have to go to the doctors to get a certificate for "having a dose".
Also consider the medical professional view is it really for them to promote people going into work to spread viruses and colds?
This isn't to say GPs are perfect, but they're not solely to blame. Yes I have come across some GPs who appear quick to issue a certificate for a week and others who are more thorough, but again in my experience we're talking about a minority over a majority.
In addition, I have come across a GPs stating quite explicitly that the patient has presented the symptoms or in some cases with the caveat "according to the patient". I've come across more than a few that have even stated that they "can find no physical evidence for the symptoms discussed" and they recommend a more detailed diagnosis that is outside of their remit.
In theory it may be easy for me to describe back pain to my GP, state I work in a job that requires lifting and carrying, cry in pain with the rudimentary tests that are available to my GP in their practice and be told it appears I'm not fit for work for a month or more.
However, it's then up to my employer as to whether they accept that certificate at face value or decide they would like a second opinion/more analysis of the full nature and extent of this limited diagnosis.
Medical Certs come with the caveat of an initial opinion from the view of the symptoms described, employers are free to have that confirmed.