I agree that swashbuckling should not be usable as an attack that harms the opponent. Most of the combat-useful instances of swashbuckling will be Absurd. For example, pinning an opponent to the wall (via his clothing); the opponent can always drop his cloak. So, it's basically a crazy difficult way to delay your opponent for a bit. Cutting an opponent's bowstring would probably be Sheer Folly (and I really like that idea!). Cutting a crossbow string would be Absurd, but sticking your sword through the trigger mechanism might only be Ext. Hard (of course you can't use your sword while it's trapping the trigger).
Another creative use I've seen was when three Level 10 swashbucklers with 1 magic sword were fighting a werewolf. If the werewolf knew who had the magic sword, he could defend himself against that opponent. So, they used swashbuckling to toss their swords from person to person, preventing the werewolf from knowing who was able to make a successful attack.
I agree that it's a problematic skill. I interpret it almost like an artistic thing. It's good for convincing people that you're really good with your weapon, without needing to be really good with the weapon. The other combat use is to recover from fumbles.
In terms of slashing the opponent's throat: no way! That requires an attack roll. On the other hand, placing matching scratches on your opponent's cheeks (while he's stunned), totally acceptable. Swashbuckling is a skill that defies realism. It's intended to replicate movie action. If your players are looking for gritty, realistic combat, I'd recommend renaming the skill "fumble recovery" and restricting it to just that.
In short, if the maneuver sounds like it would be part of a movie-promo montage, it's a good fit for swashbuckling. Otherwise, you should devote your DP to sword skill.