Jim, it's strictly a matter of comfort.
On Ouray last week, the attendants turning on the sprinklers at the end of the day were scrambling, unroped, unfixed, along the top of the walls wearing the microspikes as they turned on the shower heads at the end of the day. There was an established boot track from their activities, but would I have the guts to hang out along the edge of a 100-150ft drop on solid ice? Not so much.
Now, I'm one of those types who'd rather be over-prepared than have to turn back because I didn't have the right equipment (had to do that last year on Thunderbolt and I was not happy). I don't mind carrying the weight, and I don't necessarily quibble over ounces. So, if I think I might need the 'pons and axe, they go on the pack. Period. If there's a question, the axe goes and the 'pons stay at home. I love the usual security I feel when wearing any of my 'pons, but I also make sure to practice self-arrest whenever I can.
The WZ presents a more unique problem in that you get a higher percentage of noobs who don't know how to use the equipment, much less want to carry it. In about another month, I would expect the ringing chorus of "do I need to take this crap?" to start up both here and on the WPSMB. wbtravis is usually one of the logical heads who says, "Yes, you both need the crap and the KNOWLEDGE of how to use them appropriately" to those questions. It's not as simple a question of take them or not.
And yes, I've had to perform a self-arrest, with a full pack, and while wearing crampons, on a glacier (the Warlow glacier in Evolution Basin last summer). The axe almost pulled out of my hands and it was everything I could do to keep my feet up and not jab my front points into the snow, risking a broken ankle/leg. Whenever I've climbed Whitney in snow conditions on the MR, I've worn 'pons and used both poles or axe and been comfortable. In the snow conditions along the trail last June with my dad, I left the 'pons at home and was fine. Hiking across the Palisade glacier in fresh snow last spring, we left the crampons off until we started up the U-notch, and even then were up to our waists so probably could have left them off.
So much is terrain, current condition and climber-comfort specific, that it makes the answer to your original questions difficult. But I think wb's advice, if you feel you need them, put them on, is spot on, and I know he would tender that by adding to know how to stop when sliding.
Have fun,
L
