Here is a situation that DropBox should be able to accommodate, but I think it presently can't. It's not hypothetical BTW, it's a relatively constant hassle for me.
My office uses DropBox professionally so I have a lot of folders shared with me, many of them contain lots of files, and they also change relatively often based on the whims of other teammates. My desktop is my "main" DropBox install and I both have a lot of available storage and I frequently manage my DB archive using selective sync.
However, when I go to add a new tablet or ultrabook with a small SSD, or re-sync a tablet I haven't used in a while, it wants to download ALL of the shares I've been invited to, and all my changes to the other folders, immediately. Often I don't have room for all this on my tablet, or at the very least I have no need for 90% of it to be loaded, so I'm wasting a ton of time and possibly filling up my storage for no reason.
I try to enable selective sync right away on this new device, but it first needs to load the file index (which takes forever itself, for some reason) and then it continues to recognize my sign-on to new folders even as the button is blocked because it is "applying selective sync settings..."
In short, there should be an option to enable selective sync DURING INSTALL, so that it won't have to grind through whatever will fit from your full archive before you can practically do this. Or, you should be able to associate your shares with various synced devices through the website - essentially, central management for selective sync.