I think every trail rider should carry a large plastic trash bag somewhere on their bike or body. They fold flat and weigh next to nothing, so there's really no downside at all! You may never ned it, but since it's basically free and so easy to carry, it's almost a crime not to have one!
"But why would I need a plastic bag on a trail ride?"
Hey, I'm glad you asked, goggle-eyes! Here are just half-a-dozen reasons, and I'll bet you can come up with even more.
1. To use as a ground cloth during a trailside repair, to keep your tools and parts out of the dirt, mud, or sand.
2. If it starts raining, punch holes in the closed end of the bag for your head and arms, and you have a light-weight waterproof make-shift poncho. I once saw a rider use the bag as "clothing" after falling in a creek, while his regular clothes were being wrung out to get rid of most of the water.
3. To wrap around the wound of an injured rider to protect the injury from the elements.
4. To provide shade or rain protection for an injured or ill rider, or to wrap around and help keep the person warm.
5. Strong plastic bags can be twisted into a makeshift rope for use as a tourniquet, to bind splints around a broken bone, to fasten something onto your bike, etc.
6. Incredible but true, a plastic bag can even be used as (drum roll, please) a plastic bag! Bags are handy for transfering gas from one bike to another, carrying water from a stream to cool an overheated bike, wrapping up unwanted clothes when the day turns hot, etc.
"Where should I carry it?"
In a pocket, in your tool bag, in a smaller plastic bag taped to a fender, or even rolled up in the bottom of your airbox if it allows easy access and doesn't interfere. Use your imagination for more than just thinking you're another Dick Burelson!