The main issue being is that you think you're covered but in fact your not. A bit like car insurance, are you covered for 3rd party driving, European cover and so on. This is the issue with PPI.
For instance lose your job, you think you're covered, oh you maybe covered after 6 weeks then they will pay your monthly payments etc. There was a number of clauses such as health issues, you're covered for some issues but not others.