"Only invest what you can afford to lose" applies just as well to any complete technology as it does to something that's still in beta. 

Remember, Bitcoin is the initial implementation of a protocol.  Even if Bitcoin is a mature technology and no longer experimental, you should still invest only what you can afford to lose, because some other implementation of the protocol (yes, an altcoin) may eventually be the one that businesses settle on as being friendlier to their interests.