Standoffs are useful for light support, or more rigid support over short distances. You can get the spacing just right by using steel washers and 1/8" aluminum nuts (http://www.mcmaster.com/#93181A009).
Aluminum nuts have a much higher coefficient of friction than 1/8" nylon spacers, so things are less likely to come undone or shift, and also, they bind against the screw when there are multiple threaded things on that screw. They’re only slightly heavier than nylon spacers (.47142g vs .38606g).
Standoffs shouldn’t be used when they’re under high flexural loads and where it’s critical that they don’t bend or shift (only these two combined). We learned this part the hard way when we tried to support the sprocket for a chain bar with standoffs. It didn’t help that there were 20 doubled over rubber bands attached to it either. When that sprocket moves out of position, there is way too much slack at the bottom, but too much friction at the top for it to move freely. This will cause the arm to not work.
When you need a set screw longer than 1", you can cut your own. Long set screws can get expensive (~$1/ea for 2" set screws). To cut your own, you can cut the head off of a 2" screw (or cut them from threaded rod, although I’m not sure it’s legal, but it’ll serve the same purpose), file off sharp edges, and then fix the threads with a die (screwing the set screw’s good end into a standoff and using a wrench to twist it through the die works well), then use some round jawed (usually slip joint) pliers to remove the set screw from the standoff. Another way of holding the set screw is by using two nuts (preferably plain hex nuts, steel if possible), and twisting those nuts together until they bind on the set screw, then you can use a wrench.
Standoffs can also be used to make a very light hopper, lighter than a plastic or c-channel hopper, but I’ll leave you to figure that one out.