Dana, at the risk of diverting off topic, I'd say there are definitely situations where the disturbance of a spey cast risks scaring fish. I fish one such place, on the River Wye, on the English/Welsh borders.
It's mostly very flat, glassy water, and you need to exercise some stealth. There's no way I'd want to use a double spey on this water; a good single spey might be OK, but I'd still rather cast overhead. I find I can more easily use a lighter-than-recommended line with an overhead cast rather than a spey cast, though that may be a reflection on my (lack of) speycasting prowess. Incidentally, there are also few places where a 100' cast is needed on this water, 60-70' is adequate for many pools, and a longer cast at a more downstream angle may actually be counter-productive as the fly fishes too slowly.
I would also question the notion that wading necessarily disturbs fish. Certainly once you're in a pool, and moving down a pace or two between casts, the fish shouldn't be unduly worried by your presence provided you don't flounder or splash. I think most trout fishermen would tell you that stealthy wading can at times bring you to very close to a feeding trout. But make a sudden splash on the surface at a similar range and they'll flee for cover.
On classic ripply floating-line water, or when using a sunk line for fish lying near the bottom, line thickness and splash may not be a problem, but there are certainly places where the reverse is true.