Hi all,
the "use special abilities" is cleary not the best wording. I agree with Peter that it's placed to prevent combining abilities like:
- benefiting a to-hit bonus from another power
- doing things during movement (as not being able to call an indirect barrage [That's for the Tau special case for Orbital Barrages calls] or be used as a spotter for indirect artillery barrage [For all armies])
Here is the list of units with Fire on the Fly ability:
> Eldars:
- Warp Spider
- Bright Lance Travois (Exodite Eldars)
- Missile Launcher Travois (Exodite Eldars)
- Star Cannon AA Travois (Exodite Eldars)
> Taus:
- All Battlesuits equiped with jump packs:
- XV25 Stealth Suits
- XV8 Crisis MK1
- XV8 Crisis MK2
- Sha'vre in Crisis
- Sha'el in Crisis
- Sha'o in Crisis
I do not see any reason to prevent a unit from using a jet-pack during its movement or using its elite ability, I doubt that they turn off jump packs during the battle

Also Exodite Starcannon AA Travois should not lose the AA ability when performing Fire on the Fly attack.
The case for Tau Markerlights & Fire on the Fly is stated in the Tau Codex on page 6: "Markerlights can not be used in conjunction with the Fire on the Fly Ability."