"According to the Centers for Disease Control and Prevention, 88 percent of E.R. visits are for urgent medical needs—i.e., those that require treatment within two hours."

I think it's important to distinguish between

A) people who visit the ER for minor medical needs (things for which an insured person would make an office visit) and

B) people who visit the ER because a minor medical need has blossomed into an urgent medical need (because the uninsured person couldn't afford an office visit while the problem was still small)

There is a lot to be said for preventive care.

I think there are a few other categories. I've run into these two:

C) People who can't get in to see a regular doctor in a reasonable amount of time, so they end up going to the ER. Maybe they don't have a "family doctor" because they don't get preventative care that often, or they move frequently, or their insurance carrier keeps changing. Or maybe their regular doctor is just unwilling to "fit you in" if you don't have a regular appointment. This happened to me when I had a gynecological problem that was serious, but not so urgent that I couldn't wait a few days. I couldn't find a doctor who would see me any sooner than 6-8 weeks in the future. I waited over a week, making desperate phone calls and trying in vain to get an appointment, and finally ended up in the ER with severe anemia.

D) People who can't get off work during regular doctor's hours, and can't take sick time without being afraid they might lose their jobs. After hours clinics are another option, but in many places they still close by 6 or 7 pm, or are only open a few days a week, and there are many health issues they won't cover. I ended up in the ER with a relatively minor problem, back when I was working an hourly job, because I knew my boss would cut my hours if I took a sick day to go to the doctor.

"People who can't get in to see a regular doctor in a reasonable amount of time, so they end up going to the ER." - you are right, although it is hard to judge what is a reasonable amount of time. Especially i nthe US, people tend to err on the side of caution, often resulting in highly inappropriate ER visits.

My insurance company has a nurse triage line that you can call to determine if going to ER is the best course of action. My sister, who is covered by the same insurance, called when her baby was running a high fever, and she didn't know if she should wait or not. The nurse triage line can help prevent a lot of unnecessary trips (she ended up with the on-call pediatrician the next day, since a bath and tylenol were bringing his fever down).