Because, to quote Garth Kemerling's simple and nonetheless essential observation (http://www.philosophypages.com):
sloppy or misleading use of ordinary language can seriously limit our ability to create and communicate correct reasoning. As philosopher John Locke pointed out three centuries ago, the achievement of human knowledge is often hampered by the use of words without fixed signification. Needless controversy is sometimes produced and perpetuated by an unacknowledged ambiguity in the application of key terms.
Now, do I want to limit my ability to create and communicate correct reasoning?