As for not being able to afford it, I think it's a good investment as it is your health.
Maybe on your budget, but on many others affording it may mean going without basics in life, such as heating and food, so not a good investment at all. After recent illness in the family and on advice from my sister who died after a battle with breast cancer, I believe income protection is a better investment. If you are ill enough you will get the treatment that you need, but there is very little help when it comes to paying the bills when one income is dropped due to major illness.