To be fair, on modern systems, memory doesn't just "sit there being unused", especially if you keep the system running for a while. Memory not used by running processes will be filled with cached files from disk. Adding additional memory can result in significant improvements, depending on your workload and other hardware.

Whether it's worth the money is a personal decision, of course. It sounds like the GP doesn't use an IDE for development; I've routinely filled up 2GB even on relatively small projects just with Eclipse.