Ralph Waldo Emerson once said, "By necessity, by proclivity, and by delight, we all quote." This is true: powerful quotes provide neatly packed moxie, and can often say what we mean to say better than we could say it ourselves.
Emerson also said, "In fact it is as difficult to appropriate the thoughts of others as it is to invent." We often misinterpret the original meaning of the quotes we quote, bending them to fit our own viewpoints.
Should the meaning of quotes change as society changes, in the same way we add new definitions to old words? Sure. Maybe. But it couldnt hurt to know what the original speaker or writer intended, lest he be rolling in his grave.