I have a security background and I've found (minor) security bugs in lastpass before and yet I still use it.

I use lastpass because it is safer than what I was doing before using a password manager and most inconvenience I'm willing to put up with for my day to day computing.

I like to think of it as a convenience vs security chart of password schemes and for me lastpass is on the pareto frontier of it and happens to be the most convenient option on that frontier.