While it is natural that an herbalist keeps to the herbs he or she knows, it is ideal according to the philosophy of herbalism to use what is at-hand. If an herbalist is trained in China and learns about the intricacies of Chinese herbal medicine, they will naturally want to maintain use of they herbs they grew familiar with- even if coming to America. Likewise, if one trains on the Navajo reservation in the uses of southwest herbs, they will continue using them even if moving elsewhere. Generally, this will still produce health if done correctly. Indeed, there are several Chinese herbs and several Navajo herbs that I have come to rely on in practice. An herbalist, however, should be ready to draw on local resources. What if there is an emergency and one is not able to go to their private store of herbs, the apothecary, or a health food shop?
Today in America herbalism is largely a luxury. It is worth pointing out that in many parts of the world herbalism is primary medicine, but here it is primarily uses by those who have fallen into the trend and don't regard their condition as too serious. It is only natural that herbalists work within the context of the times. However, what about the case of catastrophic emergency? If our country really hits hard times with a shortage of fuel, electricity, and other resources, how will we tend to illnesses. Herbalists will really feel like fools if they can no longer order from their favorite companies and know nothing about the herbs growing wild around them.
A study of materia medica is important considering the above because it approaches herbs as categories. While each herb has certain unique virtues and properties, it also fits into a general category. If a given herb is not available, we can consider herbs of the same category. Consider too that particular diseases have been treated all over the world before globalization spread certain herbs across oceans. Why should we not use what is locally available?