x86_64 is pretty space efficient. The increase in binary size is small enough that programs compiled this way run faster despite the memory hit.
x86_64 binaries do have benefits vs x86 binaries on an x86_64 platform. Each duplicated library is wasted resources, and these can add up to a lot depending upon how many they are (try loading all the gnomelibs for both arches, it's not tiny).
x86_64 is also faster than x86, although not so much as to make it super obvious.
So there's basically no upside to x86 binaries.
But the problem isn't bitness, the problem is it's buggy and crashy and crappy. The lack of a 64bit compile is just a symptom of the lousy state of their codebase.