Not to mention a few tears and screams if your multiple players happen to be young siblings prone to accidentally (or not so accidentally) messing with each other’s worlds.
If you want to further compound the issue, throw in multiple players on the same machine and you’ve got a big mess on your hands. As we emphasized in our Minecraft modding guide if you load a world without the proper mods/assets then everything in that world created with those mods/assets typically vanishes and the world can become corrupted. Worse yet, if you don’t do the careful organizing required to keep your worlds separated and only loaded with the proper mods then you risk corrupting those worlds. In short order, things go from your original vanilla installation of Minecraft to a rat’s nest of profiles, shuffled around files and an overall mess. Further, many times you find mods that only work with 1.6.x or early version of 1.7.x, and where does that leave you? It’s another big hassle to set up separate profiles for not only your bundles of mods but also for your bundles of mods and your unique Minecraft versions. One thing every Minecraft modder quickly realizes is that keeping all the worlds and all their companion mods straight is a huge hassle.