Eric's example is one case where a summit pack is entirely appropriate, safe and actually preferable. I recently climbed Mt. Shasta with a summit pack and given the specific conditions of that trip I felt perfectly safe. Carrying all of my equipment on that climb would have slowed me down considerably and compromised my balance on class 3 rock and steep, icy slopes. Yeah, I could have prepared for absolutely any possibility by carrying 50 pounds but doesn't mountaineering involve a degree of risk?

The safe use of a summit pack depends entirely on the conditions e.g. current weather, weather forecast, time/distance/elevation gain to the top, fitness level, experience level, etc. etc. I have successfully used one myself on many occasions. In many cases it is just not necessary to schlep ALL of your stuff all the way the summit.

As always YMMV.