Underpowering a speaker is only dangerous if you max out the volume and cause the signal to "clip" because you don't think its loud enough.
A clean signal oscillates (goes up and down) in a smooth sine wave-like shape, a clipped sign oscillates in a square wave-like way. The problem with a speaker playing a square wave is that at the extremity of it's travel the speaker is "stopped." A speaker playing a sine wave is always moving. Constant motion helps the speaker stay cool. A speaker that stops at the end of each motion could potentially overheat.
That is why people will say it is bad to underpower a speaker, but this is not really the case. A more accurate statement is that it is dangerous to constantly drive a speaker with an extremely clipped signal.
Sorry, im tired so that might not make too much sense, but I can clarify if need be.
Eric