Hi @maxbloch, thanks for posting the question!
This is becoming a hot topic in the community.
Whether or not you see a drop in bids is not as black and white as it may seem. Some have noticed increased interest from demand for making the switch, but I would say overall it is more likely to decrease overall yield than to increase.
Non-secure ads will serve on https sites, and there will be warnings in the console for Mixed Content on the page.
To the question of whether there is a reason not to go with HTTPS, I would just say that there is a chance for a reduction in yield, and other things that could be messy, but overall there are no glaring issues with making the switch. The question I would ask in return is, what benefits are you gaining from making the switch?
Thanks again for you interest in Sovrn!