No, one reading is not pointless. It is an actually measured O2 saturation, unlike let's say an arterial blood gas, which is a calculated saturation. One reading actually tells you what the actual saturation is. A trend is also important, but one reading isn't meaningless.

92% on room air may be normal, although on the low end. 92% on supplemental oxygen is obviously abnormal. Yes there are many variables that affect the measured O2 sat, such as percentage of methemoglobin, certain colors of nail polish, blood flow to the fingers (or wherever you are measuring), body temperature, and many others. Point is, it's not nearly as simple as your post made it out to be.