Well, I’m aware of the fact many people use those applications and even some companies praise those managers, but the problem I see is an obvious one: It’s one password instead of several. If an attacker can derive this password, he has control over all passwords protected by this manager.
In theory, you could achieve a higher security through one more secure password, but in reality it isn’t so secure as the user probably won’t choose a long password if he has to enter it every time he wants to login into a service. And there’s additional code which can have bugs, in the case of password managers most bugs will be security related.
Honestly, I don’t have statistically evidence for my claims, but it seems pretty logical to me. I also think, I read about security researches of TU Darmstadt discovering several security vulnerabilities some weeks/months ago, but I could be wrong about this.
However, interesting point!