Shadow Weavers are not on my normal army list, but they're so effective against certain targets that I just have to include them. When an opponent includes a lot of lightly armoured open-top vehicles like Dark Eldar Raiders or Land Speeders, Shadow Weavers are useful for removing their transportation before it gets close enough to do some damage. Dark Eldar Wych squads are a lot less scary when they have to march across the entire board to get to you.

The other enemy that must un-entangle themselves from the razor-sharp mesh lobbed by the Shadow Weavers are Tyranid Biovores. Most Tyranid commanders have figured out by now that it can be loads of fun to regurgitate spore mines toward Guardian squads from behind cover. Spore mines- the gift that keeps on giving.

With a few good guesses you can out-lob the biovores and keep your Guardians safe. Just make sure that when you place your gun crews they are spread out enough so that a well-placed biovore template won't get them all at once.

On occasion, I will outfit my Support Platforms with D-Cannons instead of Shadow Weavers. The advantage? The D-Cannon is a strength 10 ordinance barrage. When Necron Immortals pop-in for a visit (using that wierd, Veil-of-Darkness, deep-strike thing they do) I dust them with a trio of blast templates- Necron Immortals can't get back up after a Strength 10 AP2 Weapon wound!

Blood Angel Space Marines are another good target for them- vehicles that just have to come at you. Just pick the closest Rhino (hopefully it's the one with the Death Squad in it) and scatter a little D-Cannon over it. With a little luck, that Rhino will explode into lots of tiny pieces, leaving behind a smoking crater (and no passengers!). At the very least, they should be out of your hair for a turn- there really is no good news on the Ordinance Damage Table.