So it seems the most effective use of Arrow IV artillery is to just target the ground beneath a unit rather than worrying about attacking the actual unit.
Depends what you consider effective. You'll only do 5 damage to the unit(s) in that hex.
What he said. Homing Arrow IV does 20 damage to the object being TAGged, and 5 damage to everything else in that hex. Therefore, if you TAG the hex, you'll do 20 damage to the ground, and 5 damage to any units on said ground. Not the most efficient thing to do, but there are some uses.
Why would you be able to TAG a hex ?
TAG is at it's core a fancy laser designator, right? (EXREMELY fancy, but that's not important right now.) Therefore, if you point said laser at the ground, then the ground is TAGged, and weapons homing in on said TAG dot will go after the ground.
What could would it do to TAG a hex ?
I can think of a few situations where it'd be a good idea to TAG a hex instead of a unit.
LGBs do a flat 10 damage to everything in the hex no matter what they strike in said hex. Therefore, you could TAG the hex containing an enemy unit instead of the unit itself for much easier to-hit numbers for the TAG, making it easier to strike the units in the hex.
If you have a large number of S-Gs, LGBs, Homing Arrows, and other such weapons, you can TAG an elevated hex and easily inflict enough damage to reduce it by a level(I call this creative terraforming). It takes a LOT of firepower(200 damage or so), but I'm sure you can see the benefits of being able to remove an unneeded hill hex rapidly, allowing your forces to pass through an area far faster than your enemy would expect you to do(especially if you have a lot of non-'mech units in your force, who usually hate hills with a passion).
Homing Arrows may only do 5 AE damage, but that can still be enough sometimes. TAG a hex containing small and fast units(or any BA at all), and even if you're only doing 5 damage to them, that's an easy 5 damage, as opposed to trying to hit a superfast unit directly.
Because hexes are immobile and thus receive the -4 to-hit modifier, you can reliably succeed the TAG roll from long range. This can be very handy if the area contains very dangerous enemies you don't want to get close to, or if the ordnance you're calling down has a very large blast radius, such as an orbital strike. (Because ortillery drift is based on your margin of failure, bringing the to-hit down with TAG is a very good idea even if you don't hit the target hex. The lower the to-hit, the lower your MoF is likely to be, and thus lower odds of the shot drifting very far, or onto your own forces.)
