One use for tac is gathering intel safely and having the ability to act on the intel.
1) a: take Survey to find out if/when enemy build credits come in. Hit them prior to build completion.
b: find out who has potentially dangerous builds that may require Tornado or FB to handicap.
2) Snatch News(SN) to find out who is sending and receiving aid. Inform your sabotage experts.
3) Spy On Military(som) for several obvious reasons, but use your speed to avoid potential ambush to preserve your offensive capability.
4) by using your low loss intel you help your fellow kingdomates save tpa. Since there are no thief credits it makes economic sense for the tac to gather the bulk of intel. This is especially true for undead tac.
~ I find dwarf tac particularly good at theater control as they have a more flexible economy, mystic aura to drain enemy mana/runes, and a more prolific offense. This makes them potentially good campers.
Tac is best in the hands of hyperactives(players with a lot of time on their hands). They don't have to use speed just for volume attacks, but to take advantage of enemies in compromising circumstances. This depends on kingdom doctrine as mentioned above. Given the chance to lurk and strike is ideal. ~
* a word on the strength of mystic aura: this spell is gold to the hyperactive. Because it results in auto-blocking spells you can stop barrage attacks like Nightmare, Fireball, Tornados. I've been part of Super Waves( 25 kingdom schedules waves) that have been derailed by repeated rapid recast of Mystic Aura. Along with Clear Sight you can force a mammoth effort from the enemy to chain you. Provided you have traced armies home and wave schedule you can do preemptive strikes to near nw enemies to force out of range(oor) hits. - Look at tac in a strategic light and it can be a very rewarding choice. - I've used Mystic Aura to great effect in a similar fashion, just not subject to Super Waves myself in defense.