The first time I heard this expression, a bunch of us were discussing what firearms to carry concealed for personal protection. The case was put forward that if you are going to be armed, you should be well-armed. I cannot argue with the logic there. However, there are times when it is impractical or impossible to carry anything but a small firearm and you make do with what you have. When possible, the prudent person carries enough firearm to do the job of protecting oneself and companions or family.
But when it is time to play with informal target practice or plinking, smaller and lighter firearms make a lot of sense. Isn't the same thing true in amateur radio when it comes to effective radiated power? Play with the little gun, but when you NEED or really want to get through to someome, bring out the big guns.
Sure, when the bands are "open" and your local noise conditions are low enough, you may be able to talk halfway across the world on one watt into a coathanger. The operative word is MAY! Seldom if ever will the station on the other end be using the same operating conditions. Similar conditions are very common among those who don't think they are QRP.
A case in point: A few days ago I put out a CQ on 17 meters and was answered by a weak signal in the New York Metro area. He was running 25 watts into a G5RV thrown up on his roof. I was running about 700 watts into a Force 12 C3E antenna mounted a 30' above ground level. He was having little trouble copying me even through the moderate QSB; I was constantly on the RF Gain to keep his signal above the threshold of the noise. When I switched to my G5RV -- 60' to the apex -- I could not hear him at all and did not even bother to transmit on that antenna. My point is that if I had a station similar to his, a QSO between us at that time would have been mere wishful thinking.
Until and during that QSO, I had been feeling a little guilty because I had the means and location to put up a better system that a lot of the hams I talked to were able to do. Then I had my little epiphany. I realized that those guys who were forced by circumstances -- okay, probably some of them for other reasons not particularily deserving of sympathy -- to operate minimal stations would have way fewer contacts without the stronger stations on the band. Running power and gain is not anything to feel bad about, but should rather to serve as encouragement to others to improve their stations when possible and permitted by economic conditions. In times of emergency, when communications can save lives and property or just relieve the anxiety of families far from their loved ones, good strong stations are not a luxury -- they become a necessity.