Fact or fiction?
The very term luxury defines whatever it is added to as unnecessary. Air and water are necessities. Love and affection are luxuries. But while many would argue it’s the luxuries that make life worth living, I would extend that to subwoofers make stereos worth listening.
Ok, that’s a stretch. Clearly, stereos are worth listening even without the low frequencies, but I couldn’t resist the simile between the two statements. Poetic license.
In response to the direct question of whether a subwoofer is a luxury or a necessity much depends on your ultimate goal: are you building a reference audio system or background pleasing music setup? If the former then a subwoofer is a necessity, if the latter, not so much.
I can think of no revealing reference quality system that is not full range. And, by full range, I mean the ability to reproduce credible sound pressure below 30Hz at the listening position. Passive speakers, for the most part, cannot deliver low frequencies to the listener in the average room. For that, you need a powered subwoofer. Some speakers have built in powered subwoofers, while most do not. By far the majority of systems benefit from the addition of a pair of separate subs.
One of the reasons subs are considered unnecessary luxuries is because there’s not a lot of music with frequencies below the limits of human hearing. The lowest pedal note on a large pipe organ has a fundamental frequency of 17.45 Hz. This extreme bass note, four octaves below middle C, is more felt as a rumble than heard with the ears, as are other sounds not musically related: movement of feet and furniture on wooden stages, the rumble of a subway train below Carnegie Hall, the air traveling through ventilation systems. These are all additive sounds on recordings that help provide a realistic listening experience just as you would get at the live event.
Subs are neither unnecessary nor luxuries if we’re building a reference quality system.