I think the thing is that GW don't make rules to police player behaviour. They make em for somewhat nebulous "story-telling" reasons, so far as I can tell. This is particularly true of formations and suchlike. Oh, and also to sell stuff. You'll note that all of the core groups in that guardian formation require large amounts (3+ units) of specific types of guardians - units that most Eldar players will have one or two of, but rarely 3. Oh look - let's buy one or two more and now I can use this new formation thingumyjig...
As for Wraith/D-cannons etc being Destroyer weapons, I actually don't see the problem, altho that's probly because I've never played against a Destroyer weapon

Fluff wise it fits (even a wraithcannon opens a frickin warp portal inside whatever it hits. That's gonna leave a mark, even if you're a titan) and iirc old school D-cannon had a pretty much auto-wound rule. For what it's worth, I have 15 wraith-things. 10 of them are old school metal models, and as such have d-cannon because at the time that's all they could get. 5 are the newer plastics, who all have swords, which I am quite happy about cos it looks cool. I also have a wraithlord (no gun at all) and a wraithseer, who has the D-cannon that he comes with (although the splendidly-written FW rules list it as a Wraithcannon, despite pointing it as a D-cannon. Anyway, I never use the bugger so it's all academic anyhoo. )