That would force everyone to have extra storage whenever they like it or not. Although I suppose we could have the "share bar" stay fixed when your storage capacity increases, and if you want to use the extra storage you have gained from mexes you just move the bar.
Also, in large lobsterpots, community storage in mexes only could result in nobody having a decent storage capacity, including the people who actually want to use it to build stuff super-fast (which is pretty much the only legitimate use)
I think having builders/nanos give small amounts of storage could be a better solution. You need lots of BP to make use of a large storage anyways, so might as well tie them.