Sandy,
part of the reluctance may come from the fact that the therapy also takes its toll.
Radiation and chemo both have long term problems by themselves. They are very toxic they just happen to be a tad more toxic for cancer cells; the damage they cause may result in cancers of their own later on. If you get too aggressive, you may get acute problems and/or increase the chance of treatment related malignacies. So it is a delicate balance and it is a statistical proposition (both getting cancer and treating it), no 2 people are the same.
With respect to carbo vs cisplatin issue, they are thought to work similarly (alkylating/cross linking your DNA). From what I understand carboplatin is much less quickly excreted and they will give you a much higher dose. The one thing that may be relevant in your case is that carbo is not nephrotoxic in contrast to cisplatin which is. They may be concerned with damaging your kidneys.

hope that helps.

Markus