I'm guessing rei's was replaced with voxelmap since rei's had an issue with worlds unloading, so I would suggest not using it.
This was also my guess. Direwolf recorded a bit of a conversation on the subject in one of his more recent SMP episodes. I can't remember what episode it was, but the conversation involved other stuff like garbage collection, leaking worlds and the topic of Rei's Minimap.
I still went ahead and added it back into my pack. If I run into any world problems or leaks, I might go back to VoxelMap (without all the other Voxel stuff I don't need).
EDIT:
After running a comparison in different worlds with both minimaps, I found VoxelMap to be the better fit for me. VoxelMap ran smoother with a higher FPS overall. I disabled all the other Voxel stuff, leaving only VoxelMap and liteloader enabled.