It's like with everything, there are pros and cons to them. Yesterday, I was flying in an area (taking panos) where there were tall grasses and the ground was water soaked, so I hand caught my phantom but I had to be careful as the wind was gusty and the phantom kept getting blown back toward me. I caught it ok (in between gusts) but if I had the choice there, I would have used my clip on landing gear. Like others have said, they are cheap, raise the camera up, provide a wider stance (more stable) for landing and have more give than the original landing legs, and are good for landing on hard surfaces like pavement or gravel.
Just like hard case vs soft case, filter vs no filter, Go vs Litchi, etc. they are another accessory/tool you have. You use them when needed and they are CHEAP.