The reason for x64 stepmania is to load larger libraries and utilize more RAM, more efficiently, in database tasks such as sorting, etc. SM5 themes are progressing to the point where we can do complex sorting/caching much like any other library program can (I compare it to iTunes where you can sort by almost anything as long as the metadata is present).
I know what the point of 64 bit support is. Think is kyzen added a new method to load in songs when you start stepmania which works like a dream. Even then when I open the task manager on my machine the max memory I've seen stepmania using is 1.5 gbs of ram (and that was running a high end theme). It's pretty safe to say that for now stepmania 5 doesn't require more than 4 gbs of ram even running crazy high end themes.
I mean, this is a game about hitting arrows. Does that really need to be given access to more than 4 gbs of total memory?
The 32-bit memory limit is 2GB per process.