Hosting only an executable I download and execute means the adversarial extraction of data must be contained within the executable and bypass all security from within my system. There is a window of opportunity for sending out a signal indicating the executable can not be trusted.
I do trust the team of 1Password to be competent and not evil, but there are many things that can go wrong anyway.
I remain disappointed that there is no way to set up nor configure a 1Password.com account without the web client.