Box scrapers are useful in dragging gravel out of piles and spreading it. However, I found that I have to ride the 3ph level quite a bit to get decent spreading, especially if the ground has bumps and dips. Position control is required, because the gravel doesn't spread very well if the scraper is left floating. I believe that MarkC has gauge wheels, and they sound very useful for spreading material.
I found that I get better smoothing with the scraper than with the loader bucket by fully extending the top link so the scrapper rides on the back of its rear cutter. A rake probably would work even better for smoothing, but I don't have one.
I think it was Cowboy who said that loader buckets can be used for compacting material. With a flat bucket, put down-pressure on the bucket to the point where there is little steering, and then back-drag. I will use that idea when I need to compact as well as smooth it.
Somebody mentioned that side-plates are available for blades, and also aren't very expensive. With side-plates, a blade will carry more material since it can't come off the blade ends. Side-plates should make a blade work more like a box scraper, and they could be useful when using a blade to spread material. The main advantage of a box scraper over a blade with side plates is that the scraper cuts better due to its greater weight. However, cutting isn't a requirement when spreading gravel. A blade with side-plates should work well.