There are various points of view on this matter. Basically, there are some professional traders who actively use SL, but also there are some who don't. For example, if you buy a 50$ and set a SL order on 47.5$ it restricts your loss to 5%. However, in a fast moving market where prices change rapidly, the price at which you sell can differ from your stop price. There is a common issue with SL, it's a transparency of SL. Some traders like running the stops when the price is low enougg and trigger a mass of SL orders. After the instrument is sold at a popular SL price, it reverses direction and rallies.