Ok, sorry about that. I thought that we were talking a out Scarf usage in all tiers, dreamworld included.
If we're including DW, then we have to talk about Genesect, since apparently Scarf is all he ever runs. I can see why people do it on him; Download means he gets the boost to his attacking stats anyway, speedy STAB U-Turn is great for scouting, and he's got some nice coverage. Still, since every single person seems to run scarf on him it makes him as predictable as can be, and generally you can work around it.
Which I think brings up an important point - Scarf is only so good precisely because you can't predict it. If a Pokémon literally always runs Scarf, you just mentally add 50% to its base speed and base all your predictions off of that. If a Pokémon merely
might be running a scarf, you've got a lot more work to do. The switch ins and counters for a scarfed Pokémon are vastly different for that same Pokémon without a scarf, and it's that not knowing that keeps people on their toes (and makes the Scarf bluff more effective). Even worse are scarfers with Trick - you know have more unknowns to worry about, if you haven't established the item yet. Let's take Rotom-W.
Problem 1 - Is it scarfed?
If it is, it's going to outspeed different things to if it isn't (obviously). If I can predict it getting locked into a move I can switch in the proper counter 'mon - maybe a Ground type for Volt Switch, a Water Absorber for Hydro Pump, or just Blissey to soak up a special move. If I predict it not being scarfed, I can instead use something that outspeeds and kills.
Problem 2 - Will it Trick?
My counters just changed completely. Blissey changed from being a good switch in to soak up a special move, into a switch in that will be crippled if it comes in. If I can predict a Trick, though, I can switch in a choice user of my own, or something that doesn't mind the choice item - and after it's lost its Scarf, that something might be able to outspeed and KO.
This double layer of prediciton required makes Trick Scarfers far the most threatening users of the item, in my opinion.