On the other hand, guards have been known to catch and hang up more readily in trees and bushes! Biggest problem with guards seems to be when the pilot decides to take them off. Forgets to reinstall the original motor screws and uses the longer ones that go with the guards. Driving a screw into the winding of your motors does then no good at allI've never used them. If you're planning on flying close to obstacles such as trees and bushes it might not be a bad idea.
I have tried them and I took them off after a couple of flights. They catch a lot of wind and make the unit fight much harder to stay stable. Although I don't have any metrics, it has got to reduce the flight time as the motors are running much more to fight against the wind resistance.
The each his own, I don't care for them.