I had a horse who had no respect whatsoever for leg aids and would try things on whenever she could. I could have sat there all day politely asking her to move and she never would have. Hell, I could have sat there all day kicking her until my legs were bruised and bloody and she never would have moved. How is that good? How does that teach a horse to respect leg aids (and you)? All it teaches is they can get away with not doing what is being asked of them.
I eventually bought a pair of spurs and they were a godsend. When I lightly applied the spurs to her she responded in the same way she would to me politely asking her to move, ignoring me completely. I jabbed her with the spurs once, so she knew they had the power to be less than pleasant and that was the last issue I ever had with her ignoring my legs. I tended to wear the spurs whenever I rode however I never jabbed her again, the most I ever did was lightly rest them on her sides if she was beginning to be a pig, so she knew they were there. I lost one of my spurs one day so had to ride without, I was really nervous it would be a terrible ride considering what she had been like pre spurs but she was good as gold with my leg aids, same as she was when I was wearing the spurs. For me they were more like a threat, one that she knew I was more than happy to follow through with.
I highly disagree with riders with unsteady legs using spurs, the whole idea of spurs isn't going to work if you're constantly jabbing the horse in the sides for no reason and could set up way bigger problems than a horse not responding to leg aids. I also think the reason as to why a horse isn't responding to leg aids should be examined and this is something I regret not having done with my horse. Spurs cured her being unresponsive to my leg and didn't set up any issues, but the reason she had no respect for my leg was because she had no respect for me, something she'd never learned. I eventually addressed the root of the problem and all was fine, but if a horse is being unresponsive to the leg you need to look at the bigger picture too rather than just jumping straight into spurs which can act as a bit of a bandaid to respect issues. I haven't had much experience using spurs to refine aids but I believe they are a great help there too, and if possible this is what spurs should be saved for.
The idea of using a crop in the same way as spurs is interesting and could definitely be just as effective as spurs, and I think for riders who have less than steady legs it would be a better option than using spurs as you can control the position of your crop whatever level rider you are (provided you are focusing on what you are doing, and if you can't focus on riding properly and where you are holding your crop, I would question why you are on a horse that requires the use of a crop) and the majority of horses are responsive to crops. My horse wasn't, she hated all whips other than a lunge whip as she'd had bad experiences in the past with whips. She never really got over that, she could tolerate whips but they weren't ever going to be able to be used in the same way as spurs were.
So all in all... I believe spurs definitely have a time and place but shouldn't be a riding staple for all and sundry.