it looks like they ultimately will charge for cloud backup (just like dropbox). As you note, the incremental cost of user is zero, so they don't incur a cost by distributing lots of client applications (unlike dropbox, who incurs costs for free customers that use the <2GB plan). it will be interesting to see if dropbox responds. I like the model aerofs proposes. I'm a dropbox user but I use it entirely for the file syncing across three machines - I've only once used the web access to my files (i.e. only once used the cloud storage that aerofs would not provide). If AeroFS has the same usability as dropbox, I would switch to get more storage for free.