Why? Why not?

It's a job that takes 3 seconds, and (assuming the DPS aren't derps, which I've never seen) reduces the chance of messy fights by like, 99.9%.

Don't assume everyone to be "good" especially in DF, use whatever means you have to reduce the failure rates. Same reason why people look for a 100% HQ rotation for crafting. Same reason why Careful Synthesis is better than Basic Synthesis. Same reason why people prefer to run anything with FC/LS/well known friends.

As stated in an above post, it's also not sustainable for melee DPS to drop DoTs and spam AoEs. AoE's are good only when there are above a certain number of mobs (above 3 I'd say), for all other cases, single target is faster, and the mobs don't die fast enough to let the duration of the entire debuff go on.