Um, I guess you all know that this is a troll post because of the poster who started the thread, but the topic really isn't that unlikely.
I was very recently sacked for being off sick too much, and my GF resigned rather than being taken to disciplinary measures for the same reason.
We're both awaiting diagnoses, but we both really are ill (with different illnesses). Our employers know we are ill, and have seen us very ill, but they still took disciplinary measures.
Legally, they were entitled to; there is NO protection for employees who are sick unless it's something that's covered by the DDA, which most illness aren't (though mine should be soon, yay!)
I was also once called in by a former boss at a different company to talk over my 'frequent illness.' I'd had 4 individual days off sick in 3 months, after never having a day off sick for the year and a half prior to that; each time the illness was diarrhoea, so I couldn't even get to work, let alone teach an EFL class. My boss kept saying 'you really have to get this problem investigated,' to which I replied, several times, that yes, I know, this is why I've booked Friday afternoon off for a doctor's appointment.
I'm not sure what else she expected me to do. Come in even when I had bad diarrhoea, poo on the train all the way and then dribble excrement down my leg in front of the students? I don't think that would have gone down well. It would have probably convinced the boss that I really was ill, though!
(That one turned out to be gluten-intolerance, so was pretty easily dealt with as soon as it was diagnosed).
In that job I was also not paid for time off sick, even though I was permanent staff. AFAIK, there isn't a legal requirement for very short-term sick pay either - SSP only covers absences of longer than (I think) 3 days. That means that if you have something which causes occasional problems, like, for example, being an undiagnosed coeliac, then you don't get sick pay for that time.
There are TONS of jobs where this is the case - I'm surprised that so many of you are surprised about this.
I can see the employers' POV too, though (It is difficult covering for a sick colleague), so I don't think it's actually terrible for an employer to ask how much is 'reasonable.' There is actually an official definition of 'reasonable time off sick,' (the 8 days sometome else mentioned), so it's not just down to individual opinion on how much is reasonable.