What group of consumers are they referring to when economists argue that a moderate level of inflation spurs consumption?

Let me start with an example from somewhat unrelated study presented in this TED Talk: http://www.ted.com/talks/jonas_eliasson_how_to_solve_traffic_jams. After introducing a traffic fee, Stockholm experienced a 20% reduction in traffic during rush hour. They then conducted interviews asking people how their driving habits changed following the introduction of the traffic fee, but nearly everyone said that they hadn't made any changes. The truth was that at the individual level, there were virtually no discernible change in behavior. However, in aggregate, all these seemingly indiscernible changes added up to a major reduction in rush hour traffic. For example, a commuter might otherwise behave as they normally did, but on rare occasions they might leave for work early to avoid the fee. In aggregate these "rare occasions" are happening hundreds of times a day leading to a reduction in overall traffic. In that example, people were unaware of how the traffic fee was influencing their decisions because the effect was so small. The same is true with the relationship between inflation and consumption. Almost nobody thinks about inflation when making purchases, most people purchase what they need immediately and don't consider how the price may change in the future except maybe on "rare occasions". At the individual level, inflation has almost no discernible influence on consumption, but at a societal level inflation is positively correlated with consumption. Inflation may only lead to individuals increasing their consumption by a couple of dollars a year, but across society, those couple of dollars add up to millions of dollars in increased consumption. There are a few products though that even regular consumers seem to be hyper-aware of price fluctuations. One of these is stamps. The US Post Office introduced forever stamps because the confluence of the reduction in the use of mail and the constant increase in the price of stamps was hurting the sale of stamp books. Basically, people were finding that by the time they got around to using their stamps, the price had increased, so they quit buying stamp books and just bought stamps as needed. Introducing forever stamps meant that people could purchase stamps in the present and not have to worry about the future cost of sending mail, leading to an increase in the sell of stamp books. The last time I purchased a stamp book was at least 4 years ago, but I was one of millions of people who made or will make the same purchasing decision which results in the sell of millions of forever stamps every year. Of course, there is a one group of consumers who are much more aware of  inflation: businesses. Because they are often making purchases in large  quantities, even low rates of inflation can lead to thousands or even  millions of dollars in increased costs. This encourages businesses to  make purchases early and maintain large stocks if they believe prices  will increase in the future.

Excellent question. Being the cynic that I am, I have always believed that the governments try to justify inflation because they are always in debt. Their attempt to convince people about moderate inflation has everything to do with expanding their own clout into the economy and little to do with any kind of common good.   Anyway, for the consumer behaviour part, I will differentiate between consumption and investment. As for the consumption part, sometimes I buy things in bulk when I am sure they are non-perishable and I will eventually make good use of them. Otherwise, for the consumer goods, it does not make much difference. However, while taking investment decision, I certainly take inflation into account and for that matter, I calculate the inflation rate as twice the official figures. As for the low and fixed income wage earners, they have little or no spending above the very basic necessities. Also, for obvious reasons they are unable to pay in bulk even when doing so can save some money. So they are some of the obvious victims of inflation even though the government reports that as good for the economy.