Do you *need* one? I dunno. Technically, you really only need one gear, front and back.
But I like triples.
Keep in mind, it's not just the highest and lowest gears in the range you need to be concerned about. A triple offers better spacing between gears along with a straighter chainline in different gear ranges.
The weight penalty is minimal and I find the difference in shifting quality between a double and triple to be negligible at best.