Mac OS X Yosemite using too much RAM... Why?
Free RAM is wasted RAM. It is electrically powered regardless of its contents.

Your screen shot shows 2.52gb of cache[1], 0.375gb of compressed memory and 0.574gb of swap.

The 2.52gb of cache will be assigned to processes that ask for more memory. The difference between the "new" way and the "old" way is that the contents of "freed" memory is tracked.

Apps that "free up" memory just flush the cache - meaning they cause the operating system to forget what was in memory (the actual contents are not cleared) and all requests for the previously cached items have to wait for the data to load from disk. You won't usually notice this penalty in a big way, but the system will feel slower.

Less than a gb of swap is nothing, but if you see this number increasing while cache memory is near zero (which should show yellow or red memory pressure) it is a sign that something is making heavy demands for memory.

As for compressed memory, CPUs are so fast now that its actually faster to compress and decompress data than it is to wait for data to load from disk. As memory pressure increases, the OS will compress the memory to meet demands instead of swapping.

For more information on understanding what Activity Monitor shows, see:

Activity Monitor: Determine if your Mac could use more RAM
Activity Monitor: View memory usage

1 https://en.wikipedia.org/wiki/Cache_(computing)