I did catch fish on the dropper fly, but I also spooked enough that I considered going back to the single fly exclusively. Maybe I would have spooked some of those fish without the dropper but I'm fairly certain I wouldn't have spooked all of them-the question is the balance of fish gained vs opportunities squandered.
The rationale for the dropper seems primarily to be putting a fly on either side of the carp giving it a choice of pattern and making the eat easier to see as the fish has to turn its head in order to suck the fly in. And it does work, but there are issues; the flies have to be relatively close together amd are (sometimes quite heavily) weighted often makes for a bit of a plop on cast presentations. Depending on how spooky the fish are you can be giving yourself an unnecessary obstacle.
For drag and drop the splash isn't an issue and the extra weight can help get the flies down a bit quicker - good. Drag and drop presentations are definitely where the dropper can be a real boon. Less so for dappled and cast approaches.
But then there's the distance between the flies, a 30lb carp has a wider head than a 6lber. I'm reluctant to put the flies too close together, I've found a 4" dropper tied about 22-24" above the point fly to be the most workable. It leaves the flies anywhere from 16-28" apart depending on how it sinks. This is big enough not to crowd a big fish too much but small enough that it can work with smaller carp. But it's a compromise, I regularly found myself thinking that both flies were a bit too far away from the centre of the carp's dinner plate. A slight pull would generally put the point fly in front of the fish and could result in an eat - so for smaller, say sub 10lb fish either both flies are slightly too far away or one is nice and close with the other being too far away. So then what was the point of the dropper? I suppose it does increase your chances of putting one of the 2 flies right in the zone.
Surprisingly, I didn't foul hook any fish which was my initial concern so that's good. I think I'll continue with droppers but in a limited range of situations. I've already switched to a looped dropper so I can add or remove them without re-tying. If you fish for carp, it's probably worth trying them for yourself to see how they fit your situation.