JD, it's not like it's off limits, rangers-only, or anything of that nature - people use it all the time, especially coming down the main trail. It will cut 10-15 minutes off the return to the Portal and also bypasses crossing the North Fork creek, the longest water crossing on the main trail (if you don't count about 15 flooded switchbacks from the # 23 spring in mid-summer

). It joins the MMWT proper just south of the North Fork, at the John Muir Wilderness sign. It is steeper and rockier than the trail from the "newer" trailhead to the east of the Portal Store, so it gets more down-use than up. Maybe not the best way to begin your 22-mile hike if you're carrying a heavy load or in questionable shape, but highly recommended coming down.
If I remember what I've been told correctly, the "new" trail was originally the trail for pack stock, and was converted to an easier footpath for hikers some time ago (1970s?) after pack animals were banned from the trail, and became the main trailhead for Whitney traffic.