what situations would use memory as a RAM disk be more useful than using it as a disk cache?

Answer 1

Only if you have a relatively small, heavily used file that you do not need to save.

Answer 2

2 Situations:

1) you assign you temp folder in the RAM disk, faster and you don't keep crap on your hard drive.

2) you assign your virtual memory to the RAM drive. Non sense if you're using windows XP, but IF you have a lot of memory and IF you're running a previous version of windows, it's boosting windows performance.

Om a modern computer, you don't need to use either a RAM drive or disk cache (set it up to the minimum or the default configuration is far enough)