I don't usually approach my "cardio" with goals on burning calories per se. Typically I do cardio at specific time frames and/or intensities in hopes of eliciting specific adaptions.
I was going to say I usually only use calorie measures as part of short duration efforts or during circuits, but I do have some "longer" sets of 250 over the next few weeks.
I tend to track watts, duration, and distances on various devices based on what is appropriate (as different paces alter these at different linear and exponential rates). Calorie calculations on various machines tend to take too many assumptions (so therefore aren't accurate for actual caloric amounts burned or between different devices and therefore directly tracking work like watts in those situations seems more appropriate for my uses), but as with most things as long as the goals seem appropriate and the situation is standardized (same machine/device) is used to track progress I think it should be a fine way to measure that progress.