A good thought. It sends an email on each upgrade, however in my experience it doesn't happen SO often, tops two-three times a day, depends of course on the volume of packages you use. But it's a good idea to have a daily digest, thanks for that.
My only request is that the email "digest" be configurable. Being able to set this up to notify me once per week, or even once per month would be awesome. I could spend that one time to upgrade my deps, then lock them back down again until the next one.