Now that you can actually buy them in stacks from the Sparks vendor, instead of one-at-a-time, its pretty nice if you don't have anything else to spend your Sparks on.
Especially for Summoning Magic, which can be a serious pain to skill up the normal way, or if you're just plain impatient.
Depending on what jobs you have at your disposal, farming Sparks is stupidly quick!