As I sat down to write I realized that this is not a simple question. Consider the conventional meaning of necessities (defined as must-haves) and luxuries (defined as nice-to-haves). Which category market research falls into may depend on the eye of the beholder.
Researchers (or more accurately research sellers) may want to think of themselves as producing necessities rather than luxuries. But in the consumer world necessities are also generally commodities and often sold based on price. Researchers of course want to be seen as producing something valuable, something that is worth a premium -- in other words, a luxury. So, which is it?
Now let's look at it from a research buyer's perspective. The buyer may think of research as a necessity, something that is indispensible for making good business decisions. But in keeping with the popular perception of necessities, perhaps they feel that more than one company can provide it and are hence unwilling to pay much of a premium for it. This view would support the many research sellers who complain about the commoditization of research.