[danarlow]

Yes, this reasoning is correct. Same principle applies to combination therapies for HIV, malaria, etc., where you're at risk of developing a resistant strain within the patient. Not sure why it isn't more common among cancer therapies. Maybe not enough unique targets?

[robbiep]

Not enough unique drugs, difficulty determining mutations/susceptibility of tumour to drug, high side effect profile of drugs used in isolation and higher still in combination