Well I was just surfing the internet and I read a lot of people complaining. It's a bit strange to see 8 GB of ram chewed up while having nothing running. I do appreciate the purge idea but one can't sudo purge every hour. I had already noticed once that mavericks was using up 15,6 GB of the available 16 GB and it did concern me. Is there a way to force Mavericks not to use for example 2 GB for caching files? I have a hunge that this full ram running might splash into my face. On the other hand if macOS did it right it would indeed be no reason to worry at all. Swap is zero. Lol but seeing memory builds really concerns everyone learning to program...
In your posted case, Xcode and iOS Simulator are still in RAM. The OS is keeping them cached. The cost for this is zero: the programs are already resident in RAM because they were being used. There are no requests that need the RAM they occupy, since there's still another 9 GB of unused RAM (you have 16 GB). Hence, there is no logical reason for the OS to simply abandon the cached data.
You also have 0 bytes of swap being used, and 0 bytes compressed (gecomprimeerd). So no paging has occurred, and there has been no demand to move any page out of RAM (the first demands will compress, further demands will swap). Swapping is the most detrimental to performance, but its effects are cumulative (small swap is not a big detriment).
The purpose of caching is to keep something available quickly, so it doesn't need to be retrieved from a slower source. If the OS abandons cached data simply to have free RAM, then there is no caching, and performance suffers. Here, there's still at least 9 GB of free RAM available for other processes before the OS has to uncache anything.
If you're a programmer, you should review the hierarchy of how virtual memory, caching, and swapping works. If you don't know what those are, look them up on Wikipedia.
In any case, read here for what Activity Monitor's "memory pressure" means:
http://support.apple.com/kb/HT5890
