What about people who don't have BLM? What if you only have, say, Dancer. How does a person who only has DNC farm sparks effectively? You can't even break 500 damage except on weaponskills. That's fair? I have a Maat's Cap, so, I couldn't really care less personally. But if your job can't do it, then -you- can't do it without spending time getting a job that can. That amounts to time spent leveling some job you can't gear properly without a ton of effort that you don't really care about (spells aren't cheap, I'll remind you) in order to get the job you DO care about to top shape. Hardly fair.
edit: DNC is the go-to for Dynamis currency farming (for obvious reasons) so assuming your end-goal is to have a relic to get you into the invites to the top-tier events, DNC would be the job to do. Assuming you realize that before leveling anything else and therefore monopolize on your kill/time ratio, it behooves you to increase your survivability and DD performance. Doing so on just DNC is nigh impossible though. I have a THF mule, and in 24 hours of farming she -might- get 20k sparks. Maybe. Sometimes. That's with trying to stack RoE goals to everything possible in Ceizak (including drops) and killing the Mandragoras until they become an endangered species. Your argument is that I should spend 200k+ in gil and 3 days in Bostanieux Oubliette getting her to BLM 99 so I can swarm my way through Abyssea-Vunkerl's Helter-Skelter until I can farm 50k in 6 hours so I can do this 4 times to get a single piece of 109?