That right. It's a stunt. The bane of mountain bikers everywhere. Stunts are stupid and here's why I say that:
-They're almost never a good investment of trailbuilding time and effort. Keep in mind that wooden ladders and bridges were used extensively (and very successfully) in old-growth forests in BC in the 90s and 2000s (and still today) to allow bikes to traverse terrain where fallen trees would have otherwise made trailbuilding VERY difficult. In the right place (ie BC) they make a trail exist where otherwise none would. Not so in most other places where 4' diameter downed trees aren't strewn over every square inch of forest - in the time it takes to build a little ladder (assuming you'll do the job right) you could build hundreds and hundreds of feet of "regular" trail on most terrain.
|What a waste|
-They're expensive. If you're going to do it right, you need pressure treated lumber, good quality fasteners, and real planning and tools. And a budget to replace those things every 5 years or so as they wear out. These things cost serious money. Of course, in most cases people just use scrap lumber and old nails, which leads to my next point...
-They fall apart. You really have to put a lot of maintenance into even a small wooden structure that's left outside in the elements year round. Unless you have a dedicated crew of trail workers who will maintain your stunt(s) every spring (or more often in some cases) you'll soon have an unrideable and/or dangerous structure.
-They upset non-bike trail users and land managers. There's nothing like some illegal construction on a trail to give mountain bikers a bad name. Many people who won't raise an eyebrow at some brake bumps, a berm, or a rut or two in the trail will definitely notice and be upset by a random wooden structure (especially if it's falling apart, which it probably will be) in the middle of their hike.
-They're built by morons. Half of the stunts I see get built are destroyed in the first rainstorm because they've been placed in such a way that the trail (or the structure) is on the fall line and becomes a river when it rains (or snow melts). The other half are so half-assedly built that after 20 riders have hit them over the course of a week or two, the planks have broken or fallen off all over the place.
-And last but not least: They're not very fun. How many times do you really want to ride that same line? Sure, drops are fun. Balancing on narrow things is fun too. But unless you incorporate the drop into the flow of the trail (see: Morons) it's going to get really boring really quickly.
Folks, nature already gave us everything we need to build great trails - dirt, rocks, trees, and mountains (though sometimes you have to make do with just dirt or just rocks...) The best trails work with the terrain to allow you to enjoy what's already there, and the best trail designers can see what given terrain will offer and make the best possible use of it so that you end up with a trail that rides well, is safe to share with other users, and will last with minimal maintenance. Wooden structures very, very seldom need to be a part of that process.