Well I personally haven't found much use for any filters other than ND filters and polarisers. As Ken has said, everything else can actually be done in post processing without too much trouble.
I can't really answer your questions about the technicalities of filters - I own Cokin P series filters and my knowledge is limited to just that.
But about hand holding filters in front of lenses - sure, I do that quite a bit, especially with ultrawide lenses like the Sigma 10-20 where filter holders cause vignetting. The other reason would be that it's just a quick shot and I can't be bothered to screw on the adaptor.
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