Imagine you have 4 pigeon-engineers in a group of 45-50 halbs. After your opponent's first turn, you move 2 out & you fire all 4 at their horde (1.33 should hit -- though .33 should hit your halbs since they roll 1's and 2 of the 4 are still in the horde). On your opponents turn, you probably double flee. Next turn... you move the other two into position... fire 2 (.66 should hit). On your opponents turn, you double flee again or perform redirect shenanigans. [At the same time you have a 5th engineer on a helblaster, firing away.] On your next turn the first two engineers move back into position and fire (.66 should hit). In a perfect world, you should get almost 3 template hits on those pesky elves and delay their arrival. (If said elves have frenzy, the engineers are great frenzy bait too.)
So, let's say in an amazingly ideal world you get 21 elves under a template 3 times. Wounding elves on 3+ that's about 42 wounds.
I don't think any of us actually thinks that's going to happen, but you see the potential.
However, many things can mess up this plan including: flyers, lots of missile attacks to pick off engineers, fast cav, good use of magic, etc.
So what's my point... Well, I think the engineers need another purpose beyond just the pigeons. Double flee / redirect gives them a second purpose. We should assume that they'll all die by round 6, but if we can use them to distract, annoy, redirect, and slow down the enemy, they may be just what we need.