"sharing files is arguably easier in a cloud service than on the LAN"
Sure, because everyone has a full-duplex gigabyte link to the Internet, meaning that downloading the same data again and again is not a problem. Really ?
Sorry, but no. If I'm on a LAN, then I want my data to be accessible at GB speed, not at 10Mbps. And I don't want to burden my Internet bandwidth with the data.
I just bought a Synology 414DS this year, and I'm very happy with it. You are not going to convince me that I should instead upload all my DVD rips (store-bought) to some unicorn service and use my Internet connection to watch them.
And as for security, please. My Synology is firewalled from the web. Not visible, period. How is Dropbox more secure than that ?