I didn't use spurs for many years. Then, one day I had an epiphany. I had a mustang I was training. I was getting frustrated and thinking he just was not very bright, because he wouldn't learn certain things like turning on the hind quarters and side-pass with any precision. One day I put on a set of spurs, on the advice of my dad, just to see if it made any difference. Night and day! Suddenly that little mustang was the most willing and obedient riding partner one could have. He did everything I asked with as much precision as I wanted, and nothing more than a touch was needed. Turns out he was brighter than I was, but once I had spurs on, he changed his attitude. It greatly improved our relationship and both of us enjoyed our time together much more.
As the years have passed, I have found the use of spurs in training to help immensely. Horses tend to be more willing to try, more responsive, quicker to learn, and more obedient over the long haul. Learning to use spurs properly has not only help me with my horse training, but has made me a better trainer.
I will admit that there was a break-in period for me, and my little mustang got the brunt of it. After years of hitting hard with a heel when I needed to emphasize a cue, it took some time to get the "feel" of using spurs correctly. Now it is unusual for me to ride without them. I use a medium rowel "rodeo" spur.
For those who say a spur is for one thing and not another, I disagree. A spur is what you need it to be when you need it. It can be a touch cue, it can be a punishment, it can be a nice rub against the ribs. It's no different than your calf, your heel, your crop, or the end of your reins. It is just a tool and you use the tool you feel the most comfortable using. It's all the same to the horse. Personally, I don't carry a crop, quirt, whip, or anything of the kind. Don't need it. A touch with the spurs tells the horse everything I need to communicate, when we "need to talk".
As for using spurs on the trail, there is no general rule that can cover "the trail", simply because there are a lot of different kinds of "trail ride". Try whopping your horse with the reins when you have a pack horse lead in your other hand. Sometimes a quick jab with the spurs will get a reluctant horse through an obstacle quickly and easily, that otherwise could become a major disaster, especially when you have other horses following, such as pack animals.
I watched a great video recently of a man riding an arena trail event with no bridle on the horse, just a neck rope. It was inspirational to watch them work together. No bridle, nothing on the horse's head at all, just a neck rope...and spurs.
For me, it's better to have them and not need them, than to need them and not have them. There are no "one size fits all" rules on any of this stuff.