From the Technical Service Instructions for the Shimano Ultegra cassette, CS-6600
If using the CS-6600 (14-25T, 15-25T, 16-27T) in combination with a chainwheel, use a double chainwheel. If you use a triple chainwheel, gear shifting performance will drop, and the bicycle may fall over and serious injury will result.
Forgetting the rather entertaining, "and the bicycle may fall over", and the rather silly, "in combination with a chainwheel" (What else would I do? Pull the chain with my toes?) they seem to be warning that by using one of these cassettes with a triple, there can be shifting problems.
What sort of shifting problems would result from using one of these cassettes on a 52/39/30 triple (Ultegra triple FD, 105-GS RD)? The bike came with a 12-27, FWIW.
The intended application, BTW, is when using my studded snow tires. I found last year that between the weight of the tires and slogging through slush, I spend a lot of time in the lower gears. With three and four tooth jumps between them, I never seemed to have just the right gear for any given condition.
I prefer close-ratio cassettes for three-season riding and wanted to try one in a lower range for this winter.