When cleats were relatively cheap, I wouldn't have considered using cleat covers. Not that cleats are sometimes hard to get and around $xx a pair, these cleat covers have become a standard part of my kit. Easy to put on and take off, these cleat covers will extend the life of the cleats off the bike and give the user a confident footing on smooth surfaces. These can be a slight hassle put on and take off in the beginning, but the pros outweigh the cons. The eyes for hooking these together for storage is a nice tough, but not used once you've made room in your pockets for the covers.
There are two reasons you might want these guys. The first is that they can make it a bit easier (and quieter) to walk around in your cycling shoes, because the covers provide more traction than cleats. The other reason is that they MIGHT help your cleats last longer. Cleats wear out from clipping in and out of your pedals, but they'll also wear out quickly if you do the hike-a-bike thing a lot (e.g., due to a flat or mechanical). The covers help with the latter problem, but only if you bring them along on your ride. They fit easily in a jersey pocket, or I suppose you can use the included clip to hang them from a saddle rail. I have a pair of Kool Kovers for all of my SPD-SF shoes and it seems like they'll last forever. Just remember to check your cleats every once in a while to see how they're holding up; if you have the Kool Kovers on you might not notice when it's time to replace your cleats.
Unlike the picture, the SPD cleat covers have the rear corners cut out to accommodate the two back weight bearing points so they are not as protected as the front of the cleat. Even so, the covers seem to do a reasonable job. I've had the back points knocked off before, but not since using the cleat covers. Also helps to keep dirt and gravel out of the cleat and stays secure as long as you make sure it is well attached when you put it on.
I used to think it was strange that people would use cleat covers when you can just walk on them. Just another thing to lug around, stuff into overfilled jersey pockets, or lose. However, after replacing my 4th set of Shimano cleats and discovering that the Blue ones are sometimes hard to find I decided to give these a try. I just used them for the first time on Cycle Oregon, a 7 day organized group ride where you often walk longer distances at rest stops and in camp. Aside from the fact that they can get mucked up and you still have to carry them when riding, I thought they were great, not only for protecting the cleats, but they improve grip when walking on slippery surfaces. They also clicked on easily and didn't fall off. I noticed that some riders used the shower rings the covers came with to hang them from their seat bags, but I don't like things dangling and swinging on the back of my bike. No thank you. I'd use them on longer rides any time if I anticipate having to walk any distance. I don't need them for training rides.
After wearing through my Shimano SPD-SL cleats in a year, I decided to spring for yet another bicycle accessory. These are easy to put on and remove. They also comes with a convenient clip ring which you put through the hole in the cover and the ring can be clipped under your seat stay for easy access. Not only will these save your cleats from premature wear but they will also provide traction on slick floors. I highly recommend these, just remember to take 'em off before you you get on your bike.
There is no better way to ruin a ride than to have your cleats get so caked with mud that you can't clip or unclip. It happened to me once too often - and to make it worse my friend had these cleat covers and just pulled them off and got on the bike. I carry them with me on every ride and use them frequently. Why not? They are light, inexpensive, easy to carry and do the job. They don't last forever, so I keep an extra pair around for when I need them.
These are great, they slip on easily and protect your cleats for additional wear and tear from walking. I keep them with me when ever I my cycling. Great Price as they nearly half the price of other similar covers.