Welcome to BPL, Jane!

I concur with you to some degree, acknowledging that it can be challenging to judge when to use them or not (hence this thread, perhaps). Several people have climbed Mt. Rainier with trail runners and at least one in sandals, but they also were very experienced and scouted the routes in advanced with more traditional gear so that they knew what they were getting into. There are some fairly light alpine boots out there from Garmont and Salewa that have served me well on Rainier (including steeper, less climbed routes) and other glaciated peaks. My backcountry learning experiences on snow and ice were sans crampons or boots and we faired well also, but it also can slow you down considerably and having traction devices can pay for their weight in travel efficiency in many situations. Some low angle glaciers in the Anatolian highlands, for example, have such bullet-proof ice, I would not venture on them without full steel crampons, even if you only use them 5% of the time. In Colorado, on the other hand, the soft snow often encountered can be quite forgiving.

Knowledge, judgement, and the fear of God are among your greatest tools.

Hey Bob,
I just saw your post about using yaktrax as opposed to microspikes. Were they adequate? What kind of conditions did you use them in?

Yaktrax: don't use them, they break; microspikes have completely replaced them in New England.
Microspikes: work well when it's not steep. They don't work so great on hard ice. This winter we have lots of ice/crusty snow. Microspikes work but are marginal even on fairly flat trails here with lots of hard ice. They can roll off your feet on steep slopes. They don't have enough points on the heels, so you have to be careful going down a slope or you'll fall.

Camp Magix and Hillsound Pro trail crampons. These are very similar. They should work on most light shoes. The attachment to foot is much more secure than Microspikes. The crampon teeth are longer and sharper than microspikes and somewhat shorter than real crampons. They are suitable for almost all winter hiking in New England. If a fall means death or injury, I'd want traditional stiff boots and real crampons (and ice axe). I have the Camp Magix and like them. On really minimal running shoes you might need to add some foam padding to the straps.
http://www.ems.com/product/index.jsp?productId=4134503&cp=3677347.11360109.3688336
http://www.ems.com/product/index.jsp?productId=11755949&cp=3677347.11360109.3688336
Camp makes a number of models that can be used with soft shoes.
The non-Pro Hillsound trail crampons get poor reviews (breakage).

Extending any of these 'less than full crampons' to their limits is something best left to someone with much more mountaineering experience than I have.