If you're just skirmishing then yeah maybe it is silly (although as mentioned, it does help plate carriers -- think frames for backpacks); but if you're doing something more serious, then running around in just a tshirt with a clear gun and a hicap is silly.
Some people's "thing" is just running around and shooting; for others, a physical and/or mental challenge are what they're after. So some of us do wear not only empty plastic plates but filled or training plates. Just shooting is boring: too simple, too easy. So we aspire to accomplishing a specific objective, to rise to the physical challenges involved in doing so.
Nothing worth doing is easy. (but not everyone subscribes to that)