It might be worth comparing, say, 5 hypothetical war walkers with Bright Lances, to 4 hornets with Bright Lances, if we want to compare point for point. Those two groups cost the same. So for the same cost, War Walkers are putting out 25% more damage output in Bright Lances. They are less mobile since they can't take CTM, but they get the scout-move. How do these units stand up on average to certain kinds of fire?
3 Meltagun (half range) Hits
Vs War Walkers: 2 wounds, 4/3 after saves, 6 damage on average - enough to take down 1 War Walker
Vs Hornets: 2 wounds, 2 after saves, 9 damage on average - enough to take down 1 Hornet
2 Lascannon Hits
Vs War Walkers: 4/3 wounds, 8/9 after saves, 3.1 damage on average - enough to take down about 0.5 War Walkers
Vs Hornets: 4/3 wounds, 10/9 after saves, 3.6 damage on average - enough to take down about 0.5 Hornets
2 Plasma Gun Overcharged Hits
Vs War Walkers: 4/3 Wounds, 8/9 after saves, 1.7 damage on average - enough to take down about 0.25 War WAlkers
Vs Hornets: 4/3 wounds, 10/9 after saves, 2.2 damage on average - enough to take down about 0.25 Hornets
9 Assault Cannon Hits
Vs War Walkers: 4.5 wounds, 3 after saves, 3 damage on average - enough to take down about 0.5 War Walkers
Vs Hornets: 4.5 wounds, 2.25 after saves, 2.25 damage on average - enough to take down about 0.25 Hornets
18 Bolter hits
Vs War Walkers: 6 wounds, 3 after saves, 3 damage on average - enough to take down about 0.5 War Walkers
Vs Hornets: 6 wounds, 2 after saves, 2 damage on average - enough to take down about 0.25 Hornets
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So basically, thanks to the Power Field, War Walkers are about as durable as Hornets are, against dedicated antitank weapons against which the Power Field actually matters—it makes up for the worse armor save and lower wound count. However, small arms fire and anti-infantry weapons are twice as effective against War Walkers as they are against Hornets, since the Power Field doesn't do anything against that.
So, if you want a platform that's more durable against small arms fire and more mobile, the Hornet delivers that to you. However, if you want a platform that gets more firepower, point-per-point, War Walkers give you about 25% more firepower in return for being much worse against anti-infantry guns (though still reasonably sturdy against them)
As a minor note, the superior BS and S of the War Walker could at some point be useful as well. The Hornet is effectively incapable of dealing damage in Assault except via extreme luck. Three War Walkers, on the other hand, will kill about one Space Marine on average in each round of assault, which while also quite bad, isn't nothing. I wouldn't ever use them in assault but they are at least capable of it.
I think that the Hornet being generally more durable is a good case for it. That being said, I own War Walkers and not Hornets, so I'll be using War Walkers