Yeah, deep striking them, at least to me seems like a poor way to play them. Their low numbers and no exarch powers like herald of victory means they wouldn't be accurate the turn they come in, and given the wording of the Spectre jet packs, they won't get their cover save (since you need to "move" in the movement phase to get the initial 5+ cover).
Personally, I'm planning on playing them as defensive warp spiders. They have longer range than warp spiders and their weapons are more specialized, so since I play warp spiders all the time, the shadow spectres would fill a scalpel role were the warp spiders are more general purpose.
Ill probably end up running them as 5 with an exarch sans exarch powers (although monster hunter is a tempting option). Most of the exarch powers require them to get in close with the enemy, and even with a 4+ cover, i feel its too big a risk with the low body count and possibly of getting charged.
Illiryth tho, thats an entirely different story. He's great at range and in combat, and can make great use of those exarch powers