There is very little incentive to then turn around and use your SP to flag a post where you get nothing in return. There would be some incentive if you still got a curation reward for flagging, but I don't think that's the case.

On a side note here: there's @steemflagrewards, rewarding flags with an upvote with slightly more value. The project is comparably young, still rather limited in terms of SP and by far no compensation for whale flags, but it's a great incentive for "small players" to use some of the stake also for flags.