Is there any usable synergy for the winter elves between their world spell and the raider trait?
Is it worth producing a couple dozen units mixed between pillagers (horsemen maybe?) and stack protectors (hunters or mages maybe), casting the world spell to turn them into pirates then going on a pillaging spree?
Do people do this? is it effective? Is it a total waste of hammers?

I.

EDIT: Oops- i meant: Svartalfar

Get a couple of archmages/liches with shadow 3 and spell ext. 2and you could pillage your friends without any risk whatsoever.

Is it worth producing a couple dozen units mixed between pillagers (horsemen maybe?) and stack protectors (hunters or mages maybe), casting the world spell to turn them into pirates then going on a pillaging spree?
Do people do this? is it effective? Is it a total waste of hammers?

The micro, because you cannot group HN units, is unbearable (even in SP). If you have the time, just use whatever units are sitting around - of course it's worth it to throw in a couple horses. I think the only other time to use it is in the early game when you can kill workers without retribution.

There is a lot of micromanagement but sending in a veiled HN stack can inflict a vast amount of damage without having to declare war. I prefer waiting till after assassins to do this. The best part is that HN units don't cause you to gain any WW. It's important to stage attacks initially from out of defensible terrain and have some good stack defenders (hunters/swordsmen)

I think the best use of their world spell is early. Steal Orthus Axe from the wounded schmuck who beat Orthus, or kill that warrior/settler that is moving towards your prefered city location.
Later on you can probably build some recon units with Esus religion for another round of friendly fire.

The key is micromanagement. It's all preference as to when you use the spell, which units you use, and how you use em. But micromanagin it well could get a large amount of gold and cause significant damage to your rival w/o actually declarin war. I find it's fun to send in a decent sized army with HN. Then after they've done a lot of damage, declare war and send in the big guns to start capturin cities.

As an alternative, which I've had a lot of fun in Marnok's mod/raging barbs with lately, is to build Duin, Kithra, Alzakhan, and a bunch of Satyrs and Rangers, then use VoN. You can quite literally wipe out every AI defender in a matter of turns, then sit back and let the barbs conquer the world for you. Maybe not strategically optimal, but it sure is funny.