Some of the plugins (which is a big part of choosing IDEA over the sub-products) aren't backwards compatible (I had issues with the golang plugin in 12, but it's fine in the community edition of 13). Right now I've got PyCharm 3.4 (work license), IDEA 12UE (personal license), IDEA 13CE, and Android Studio on my machine to support the various languages I'm using on somewhat regular basis (90+% of my time is spent in python, which is why I don't just use the python plugin for IDEA - the plugins are nice, but the context menus get crazy one you have more than one active at a time)