It's perfectly reasonable for you to decline to provide that information. It's also perfectly for them to decline to provide employment (or consulting business) on the basis of it.
I also don't like to provide these numbers; my usual (in my view, perfectly ethnical and justified) dodge goes as follows:
(1) I give them 1 or 2 numbers that are easily verifiable -- and which I don't mind being "leaked" -- e.g. FT salaries at bigcorps in the past few years. This is in accordance with basic game theory / negotiating strategy (whereby if you make a partial concession to people, they feel they have an "out" and and can save face, rather than press you for a full concession to what they were asking for).
(2) As to the rest, I say "my clients / employers have asked that this information be kept confidential, and I'd like to honor their requests."
Your suggestion to raise the issue of unintentional leaks (due to concerns about cloud security) is quite excellent, and I may incorporate it into my dodge strategy, also.
The bottom line is that most of the people asking are basically drones who don't really know why they're asking, just that someone told them to (or they have some vague sense that it will provide them with some kind of negotiating leverage).
At the end of day, you don't have to play their games -- but keep in mind that you'll incur a 30% or so automatic rejection chance for doing so, across the board. My gut feeling is that companies that are sticklers aren't really all that desirable to work for, anyway.