I think I may have found a way to prevent FCP from gobbling up all available memory (and bring my mac to its knees).
The idea came to me from a post by Tony Reidsma in this thread: https://discussions.apple.com/thread/3770230?start=0&tstart=0
He pointed to this page regarding memory usage in FCP X and Motion: http://bradbell.tv/filmmaking/improving-ram-performance-in-final-cut-pro-x/
The above page,mentions a preference setting that exists in Motion but "not in FCP X" - the setting for a "cache percentage".
This is supposed to determine how much of the available memory Motion should retain for caching content (or so I gather).
Sure enough, this preference is not there visibly in FCP X, but since FCP X and Motion share a common foundation, I thought it might be there anyway.
So after looking into the plist files in XCode, I found the name of this preference setting and used the defaults command in Terminal.
I have been throwing everything at it for a day now and it seems to be working well, so I thought I'd share.
Here is the relevant defaults command (NOTE: use this in Terminal when FCP X is NOT running):
defaults write com.apple.FinalCut OZPreferenceManager::CachePercentage -int 20
(this seems leave free about 20% of the ram available when FCP X starts; I have experimented with different values, and apparently the higher the number, the more free memory is untouched)
I have 15 applications running in MacBook Pro (with 8GB Ram), for hours, and no memory problems (and no need to "purge"). And FCP X and all other applications have not shown any of the slowdowns typical of low memory situations.
NOTE: While I have not experienced any negative effects, I have no internal knowledge of the software and no guarantee that this is effective or harmless.
One can revert the above setting by typing (or pasting) the following in Terminal (again, with FCP X NOT running):
defaults delete com.apple.FinalCut OZPreferenceManager::CachePercentage
If any of you try this and it does or does not work for you, I'd appreciate if you post your results back here.