I'm not talking about the possible unpleasant experiences during the chemotherapy - I've read up on these. I'm more concerned with any longer-term negative effect on health and ability to fight the cancer.

For example, I've heard that chemotherapy can damage the immune system, the very thing that's needed to fight the cancer.

And I've also seen suggestions that diet is very important in boosting the immune system, but I've read that chemotherapy can damage the villi which line the small intestine, so that some nutrients can no longer be digested, and malnutrition can result.

So, for a colon cancer patient who has undergone surgery, is adjuvant chemotherapy necessarily a good "insurance policy" to reduce the chance of cancer returning in another part of the body in the future?

-Chemotherapy certainly suppresses immune function, but a) this is temporary and b) if you have developed an invasive cancer, the immune surveillance system must have already failed. No diet boosts the immune system. Again, any effects on intestinal absorption due to chemotherapy is very temporary. It depends on the stage. There is certainly good evidence that for T3 primary tumours, and node positive tumours, adjuvant chemotherapy reduces the risk of recurrence.

-Except in certain desperate straits it is unusual for chemotherapy to be applied without good evidence from multiple controlled trials that it improves survival length or, in many instances the long term cure rate of the cancer. .

This proves that, as Steph has said, any temporary immune suppression is irrelevant to "fighting the cancer", as you put it. It is unfortunate that sections of alternative medicine to obsess about this point, in disregard of readily available evidence.

It is true that later malignancies are more common in persons having probably more aggressive types of chemotherapy, but this is not necessarily an immune effect and is just one of several costs that cancer cure can entail.