You can't detect a priori whether data is "unused." If you guess wrong a few times in a row, you get the familiar pattern where your Linux box is unresponsive to everything and needs to be bounced.
If you could detect whether data is rarely used, swap still isn't necessary. Applications can mmap() a file and use that region for "rarely used data" if such is known in advance.
Extraneous functions should be backed by the executable in the common case. In the JIT case, they probably won't be JITted anyway.
I still think the OOM killer is less intrusive than swapping to disk. It kills some, but not all, of the processes on the machine get killed. The system pretty reliably comes back to life in less time than it takes a human to diagnose the problem and bounce the system. As a bonus, no human needs to get involved.
