Scarf Sawk definitely isn't inferior to Band Sawk, it's just a lot easier to wall. Quagsire, Exeggutor, Gurdurr, Golbat, Seadra, bulky Torterra, defensive Luxray, offensive Tangela, Hypno, Meganium, etc. all avoid a 2HKO from Scarf Sawk. There's much more than that, of course, and Scarf Sawk can't just spam Close Combat like Band Sawk; get rid of the few fighting resists through Earthquake/Stone Edge and you're set. In fact, the frailer resists such as Rotom-S and Altaria get 2HKO'd by Choice Band Close Combat.
I've found that it is overwhelmingly easy to just run through many teams with just Band Close Combat. Many people think that Pokemon such as Rotom-S and Muk will save them, but they're wrong. Having a Sawk weakness is one of the worst things you can have, especially because of Sturdy. Luckily, it's countered by quite a few Pokemon; Musharna, Weezing, Misdreavus, Duosion, Amoonguss, Tangela, Vileplume, and a few other Pokemon all counter Sawk. A few however are overcome they have taken some prior damage and Sawk is Adamant.
But anyways, Sawk is really annoying to beat without one of those Pokemon. Sturdy doesn't help either making you have to set-up SR/Spikes/whatever so you can have a chance at OHKO'ing it. Sawk deserves all the usage gets and potentially more. It may be countered by quite a few threats and is not very fast, but it can easily rip through teams thanks to its insane power; you just need to remove those threats. Watch how easily teams crumble to Choice Band Close Combat if they don't have any good resists. (Good resists as in Pokemon that can actually not get 2HKO'd a CB Close Combat after SR)
However, I wouldn't go as far to say that it's broken. It's very powerful, especially if you lack good resists, and it's one of those Pokemon that you NEED to prepare for, but it's not overly challenging to prepare for it, nor does Sawk wallbreak too well.
@ChaoticaMortis: Sawk can really threaten weather offense teams as well. Usually they don't set-up SR right away or at all so Choice Band Sawk can deal some serious damage to them when Rain Dance/Sunny Day wares off. I know I had a sun team that struggled with Sawk; I even had to give Exeggutor a lot of extra Defense EV's to deal with it. Part of the problem with beating Sawk was identifying whether it was Scarf or not, then I had to send out the appropriate Pokemon to beat it. Personally, Scarf Sawk was much easier to deal with on that team.
Also, the answer to your last question is simple: Sawk is a powerful wallbreaker right from the start that doesn't need to set-up or other Pokemon to set-up for it to be devastating. Try out Choice Band Adamant Sawk for a few matches. You'll be impressed by its power.