Yeah, Peter(and Alex); but Andrew is emphasizing that the added resistance not only loses some power(at a given volume control setting), but more importantly results in an uneven frequency response. It's the same principle that applies when a speaker wire is too thin and therefore has too high resistance for the run involved.
Resistances connected in series form a voltage divider. For example, if a 4 ohm resistor was connected in series with a nominally 4 ohm speaker, the resistor and speaker would divide the voltage equally at a frequency where the speaker resistance actually was 4 ohms. In practice, speakers can vary widely in impedance as the frequency varies. If the impedance was say 12 ohms at a certain frequency the speaker would take a larger "share" of the voltage at that frequency and would be relatively louder than at the 4 ohm frequency. So, the speaker response would go up and down as frequency varied instead of being relatively flat.
Speaker wires with too high resistance or amplifier designs(e.g., certain tube designs with output impedances of several ohms)with high output impedance are examples of practices that can lead to flawed sound.
