See that seems like a bit of a downside to me, I like being able to pull down things from my Sparkleshare server onto my laptop when I'm out, but I don't have my computer on all the time (Infact, most of the time, when my laptop is on my computer is off and vice-versa). I still think it's a great idea but having some cloud storage is a definate plus.
Like I say, I use Sparkleshare instead of Dropbox and also have a Drobo for central storage, this isn't something I want to do because I can already do it, I was just pointing out it could be a problem for some people