I edited that as you responded. I'm just saying, Future Sight diminishes Slowking's ability to function as an individual mon. If it's up against something where Psychic coverage is useful, and it has Future Sight, it becomes easier to play around.
Not to mention, if you're up against a poison type w setup, it can set up in your face then max guard on the Future Sight. Fighting type w Lum Berry? Free setup.
Overall, I've hardly run into situations where Future Sight has been effectively used against me. If someone uses it, I can either set up on it or benefit from not taking damage t1 while getting my turn in, then send in something to absorb it t3. In the latter scenario, my T1 becomes more valuable than theirs. T1, I do something and they set up a move for the future. T2, we both do something. T3, I send in something that I know will do well against Future Sight, because I know it's coming (or I choose a move that responds well to Future Sight), and they do something. I get a full turn T1, but their turn in T1 is often diminished because I can respond to it with certainty, as well as retaining whatever choice I make T1. If they had clicked Psychic/Psyshock T1, I would receive full damage. Even if I switch T1, they have a greater advantage here because they their turn T1 is not as likely to be diminished. In situation 1, the one on the receiving end of Future Sight receives 3 full turns. The person using Future Sight receives 2 turns, and a turn that is more likely to be diminished since the other person knows "Psychic move guaranteed incoming." If the person simply had Psychic/Psyshock in a useful situation t1, then you're thinking "worst-case scenario: they will spread status or deal damage with Psychic this turn." You're not thinking, "worst-case scenario: they will spread status or deliver a psychic type move on t3," in which case you can use t1 more freely, which is useful if you have something that can take advantage of no-Psychic move t1/t2. Ex. maybe they're using it on my Qwilfish, and I can now set up two laters of spikes before switching.
And while you do have to account for the possibility that Slowking has Psyshock/Psychic instead of Future Sight, scouting can reveal this. And, again, it is detrimental in 1v1 situations where immediate damage is more useful.
And I do see how you can flip that on its head, and T3 send in something that will have an advantage against whatever is sent in to respond to Future Sight. But due to the randomness of the format, I have yet to encounter that situation (except, maybe, one time out of multiple).
I feel like I'm explaining this poorly, but yeah. I'll definitely mess around with it more, and maybe I'll find it more useful in terms of baiting switches in the future.