I guess this line of thinking starts with sunlight and water...you can't stay alive without them, but too much and you'll drown or die of skin cancer.

I remember the Dr. Ames "mutagenicity" index, where he simply put things in a petri dish and watched their effect on bacteria, etc. It seemed that everything was "mutagenic" to some degree, some more than others. Since as you say, many plants have evolved defenses to survive (ie to kill viruses and bacteria), they too are "mutagenic" at some level, but generally less "mutagenic" than things like char-broiled red meat. Variety and moderation seems the key to good health.