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.