If you need it for the extreme temp ranges that's one thing, but if you just need good performance, anything more than the Ultra II is a waste of money, the camera isn't fast enough to take advantage of faster cards.
That is a good point. The cameras **currently in the market**, are not optimized to take advantage of the speed of any of these cards. In fact, Rob Galbraith's site had some detailed objective tests taken with several of these cards, mated to several high-end dSLRs and it showed that the high speeds of these cards are never used.
But, it does not mean that the cameras currently in the pipeline, would remain the same. Why would companies maintain the status quo, if they can speed up the file-write-to-card process (enabling either a smaller/cheaper buffer or extending the number of files that can be written to the existing buffer) and claim a competitive advantage in the marketplace ?
Methinks, with Extreme IVs with 40MB/S *Read AND Write* speeds available, it is only a matter of a year or so before cameras that can take advantage of that speed, will hit the market. Memory manufacturers like Sandisk, work hand-in-hand with camera manufacturers and if they come out with such a high-speed product, the writing on the wall is that cameras that can take advantage of the speed, are definitely in the pipeline.