We're doing the downloaders because it let's us do p2p, which is how we can offer free and unlimited. It also affords end-to-end encryption, and resumable/restartable transfers.
We think we can mitigate those concerns some by code-signing the exe, having the download from https, and building a recognized brand.
Also, we're considering a feature called "Channels". After you set up a channel with your grandpa, it can be trusted. You just drag files onto your "Grandpa Channel" and they go to his HD in that channel's folder (he gets a pop up to accept/decline). This also ends up being a convenience for frequent sends because you don't have to enter your recipient's email - you just drag and drop files onto the channel and they start going.
Does that address your concerns?