Anyway, the point here is to lift a rules issue that either lacks a clear-cut solution, or that has a solution that sort of baffles me, sense-wise (as in it makes none). Enter the rules for vehicles shooting.
In general, the vehicle rules modify some of the general model rules, but leave others intact, yes? So, if a rule isn't explicitly changed by the vehicle rules, it still applies to vehicles. This will be the basis for the argument.
Now, we turn to page 27 of the Big Black Book. In the third paragraph under "Type", there is this little gem of a sentence:
"Remember that a player can decide that any model in a firing unit is not going to fire his weapon. However, if a model does fire, it must do so at full effect"
So what are the implications? Well, first, I take the first sentence to be a simple yes/no; so, you either fire or you do not. The bit that says "it must do so at full effect" has implications for vehicles (and Monstrous Creatures) beyond simply preventing one from firing a heavy bolter at half rate, or whatever. A model that fires must shoot as much as it can.
Now, for most vehicles, in most circumstances, this matters little. Oh, so I have to fire my Storm Bolter at that vehicle I just shot with my Lascannon? Boo-hoo. Pling, pling. For some, however, the implications are dire. Point in case: the Dark Eldar Razorwing. Comes with four one-shot, large blast missiles. Can fire any and all weapons while moving 12". Has two Dark Lances.
Problem the first: if the Razorwing decides to fire its Dark Lances while it still has those missiles, they're going to be fired too, quite possibly at a target that will just shrug them off.
Problem the second: when the Razorwing fires its missiles, its all or nothing. Loose one, and the rest will follow. No shooting some now and saving some for that other large mob of infantry targets.
The same obviosuly applies to any vehicle mounted one-shot weapon (Hunter-Killers, Bloodstrikes, possibly others), but the Dark Eldar fliers are extra vulnerable due to that usually quite nifty ability to shoot full pelt at cruising speed, which means you can't even limit yourself to one weapon by moving fast.
And with that, it's Incarias out.