It's also important to keep in mind that, just because something has therapeutic potential doesn't mean the chemicals are in a concentration that's helpful - and not useless or even toxic.
Oh yes, I agree with this. However, for most of these plant chemicals, I believe the concentrations made available in doses (standardized preparation) shouldn't be too toxic for human use.
Most phytochemicals are intended to protect the plant against being eaten.
Please, can you help me understand why you used "most"? Because I am sensing some high level botanical knowledge in that statement.
Well, I only have basic botanical knowledge, as my degree focuses more on human - and molecular biology. But phytochemicals are generally produced to help the plant thrive, be it to avoid being eaten (most uses) or kill other plants nearby (that's not too rare either).