One such case, compiling software. If the resulting files of compiling are stored on a RAM disk and the final exe copied out when the compile is done, the entire compilation and linking process will go much faster.
Actually, compiling code is a great counter-example. Compiling requires lots of burst RAM when building trees for each individual file, and almost all languages are heavilly memory-limited rather than disk-access-time limited, meaning that each compilation unit puts a fair whack of pressure on the pagefile if you don't have enough RAM available for it.
By installing a RAM driver, you artificially reduce the amount of RAM that's available to your machine (since lots of RAM is currently parked holding temporary files that you're not looking at), causing your programs dealing with the trees that you are looking at to violently thrash the page-file to get memory available for the compilation process.
Even if that wasn't the case - the filesystem driver keeps recently written files in memory anyway, so your stated performance benefit because "those obj files are in memory" you get without the RAM disk anyway.
As a general rule, RAM disks don't help you more than they hurt you. There are cases where they work - but they're few, far between and generally pretty contrived.