Why?! Surely this is the optimum time to provide source code, given the amount of trust one has to place in these things to not lose/misplace your data.
Especially when crypto is involved. Given the large number of completely broken cryptosystems, making this open source will give it a fighting chance to not be entirely broken.
Anything that handles critical data and sends it encrypted to third parties needs to be open source in this day and age or wont be used by people who take their data serious. Actually it doesn't even need to be sent to third parties to hit the open source requirement. Having crypto involved like you said is reason enough.
As long as 'free for personal use' means 'closed source' then at least I would have a hard time trusting a tool backing up personal files into public cloud storage without transparency about how the crypto is implemented. I suspect that may also be the case for much of your 'personal use' target audience.
A thought: Do you think people would pay collectively (given the chance) like a fundraiser to publish the source, when a certain limit is reached? Maybe even pay the developer's bills that way? I wonder..