They do offer selective sync which can be configured on a folder level in the settings. I've been using this for a while now to save space on device like the MBA with limited space on solid state drives.
It's not the same, if you don't sync the folder then you can't easily add to it or view the contents? On mobile you can add and view files in unsynced folders easily - they just download on demand.
Selective sync as in I'm syncing one folder at ~/a/b/c, another at ~/g/h/j and at /whatever/ etc etc?
As in on device X don't sync folder Y. If you have a photos folder with 100GB of images you can set it so that your MacBook Air does not sync that folder but does get your other Dropbox folders.
I'm pretty sure SpiderOak offers this feature, as well as an option to not sync specific folders on specific computers.
I have a Dell Mini with a tiny SSD, and I use selective sync to only sync my Documents folder. I then symlinked ~/Documents to ~/Dropbox/Documents. It works quite nicely. (and I do sync a few other folders, this is just an example)