I don't normally use locusts, but I have studied their use through allies and through enemies.
If you think about it, most building are worth more than what they build, and if you take out those buildings, not only can the enemy not produce those units, but he/she just lost out on resources.
The ability to level citadels and fortresses to the ground is highly prized, and to do it from long range it a bit OP. Cobras kill vehciles temporarly. Locusts seal off the entrance vehciles, infantry and air units come through into the virtual world. Make you judgement now, my freinds, and what is more of a high priorty target: the unit, or the units mother?
Human: Regret? A phrophet? Is this who you call your saviour, your leader? This aberation of self preserverance and spite? The dulutant who manipulates the destineys of a hundread races for his own deisires?
Covenent: Um, yes?
Human: Cool! Im in!