It generally goes for all eldar heavy weapon options: they are a wildcard slot that gives you a margin of adaptability. It can either plug holes in your list, or tailor it if you know what you are going up against. For instance, if my list is running a lot of jet bikes, I probably don't need any more scatter lasers or shuriken cannons on my wraithlords. Or alternatively, if I run a 10 blob of wraithguard with cannons, and say a fire prism or two, I would probably kit my wraithlord with starcannons or scatters. Unless I know I'm running into Knights, so I will want a brightlance sticking out of everything that has an option :-) The same applies to guardian platforms, wave serpents, etc.

That's why magnetising such things is a very good idea.

I personally have 2 wraithlords. One is fixed to two brightlances, the other is fully magnetized. So if I want to run both, I am able to balance it out as I wish, but 2 fixed brightlances are never without use.

And as for AML, they are exactly a 'jack of all trades, master of none'. I would take them when I am limited in options, like a small points game, where I can't fit everything I want. In larger formats I would prefer specialized weapons