Hmm, so you admit that you use Devastators as an alternate to AA units then? Point made surely.
Ok, against the Eldar it's good (maybe too good) but really you should have bought more AA units. Are Scorpions all that the Marines get or do they have a Whirlwind Flak unit too? I forget. I've used Tarantulas for Anti Pop-Up work before, they can be pretty good.
So, Snap Fire Works against the Eldar, yeah? Well, just consider what that Snap Fire rule is doing to a slow moving foot slogging army each turn. Not only do they have to face the normal two to three turns of fire but then they have to face Snap Fire from the units they want to engage. Yes, I could buy lots of artillery to soften them up but you see I wanted to play Orks or Chaos not Imperial Guard.
Personally I don't rate Hydras that highly (12 attacks at 5+ with -1TSM) a 33% hit rate is no big deal, the range is formidable but still I'm not impressed, I'd be more nervous having my Titan Shields taken down with this than my Infantry and it's easier to shoot artillery at exposed vehicles in a three unit squad than a host of infantry in cover, BP should be considered too. Firestorms would be Anti-Tank scary though but three Fire Prisms would do almost the same except for the range and AA Snap Fire ability.
PD is Point Defence for protecting the unit from assault not for picking stuff off as it goes by. That's what the weapons with the stats are for. ÂÂ

Is this what Snap Fire should really be used for or what you hope it should be for? (Anyone see what I did there? I took Screams words from the Pink Horrors argument and turned them against you. Nice eh?)