In some areas of the world, salt water fly fishing while using a fly without a weed guard can make things frustrating. Every time you retrieve your fly you yank up a flats version of a creaser salad all wrapped up around your hook.
If your fly grabs some of this salad as your fishing/stripping your fly odds are really good that the fish will lose interest quick and move off.
I am a huge believer of weed guards. I think they are imperative. I have learned my lesson the hard way unfortunately.
I love a double weed guard. This way the two sections can lay down on either side of the hook point when the fish crushes down on your fly. I have had many single weed Guards actually get stuck on the hook point during a hook set and I lose the fish. As long as your weed guards are strong enough to actually push your fly away from the grass you're in good shape.
In this picture above you can get a good look at the double week guard action. Learn it, use it Love it!
Here is a video link on a great way to tie on a weed guard here!