Guide: (Very) Basic Battle Tactics

[ano]

No doubt that it is worth the cost in case of deflecting shadow blasts or other area spells. But, in my opinion, counting chaff as "easily expendable" is a mistake. No troop is expendable unless it eats a lot of upkeep.
For example, maenads are decent troops that cost no upkeep and thus are (and that is not obvious!) more valuable than, say, satyrs or regular infantry. If you plan to have losses, upkeep cost is a serious factor to take into account when thinking what to use as a arrow\spell decoy.
Also, later in the game with many buffs in serious battles initial quality of troops means much less than in the beginning, and if you managed to keep enough "worthless free chaff" alive, it will help you much.