in this case, IMO, convenience == enhanced, unnecessary risk. As stated earlier, ANY breach in the convenience and all your accounts are instantly at risk.
I've choosen to protect my assets, but you can assess your risk (as it should be) for yourself. Notice --- IMO is all over the subject.
It's not like that. You gain some with a password manager while you lose some, and the convenience is not the only thing you gain. You gain some in respect of SECURITY, even though you still lose some in repsect of SECURITY. Usually, you gain more security than you lose with a password manager, and that's the reason why many people recommend a password manager.
The single worst thing you can do is password reuse, because your password (either in plaintext or hash) is almost guaranteed to be leaked somewhere. FYI, big players like Adobe, Dropbox, LinkedIn, MySpace, tumblr and Zynga have leaked anywhere from 65 million to 359 million accounts and password (hash)s. Once your reused password is leaked in plaintext (or recovered from leaked hash), you have whole bunch of compromised accounts.
You can deal with this issues probably in three different ways.
1. Tiered password reusing structure (such as pswd1234 for weak websites which can't cause you any harm to you even if it's leaked, lrfebjdbwhwz for medium websites which can cause you some harm but not grave harm, oIiskIFoy8@uHmOl%k&!%w@Vg for emails and bankings)
2. Salting the strong base password for each websites (For example, if your base password is P@V0sYz$@%9ufMEOZI|1I|jIljl|iIlI|ljIjii|Ii|li1jlI,
TcSoP@V0sYz$@%9ufMEOZI
.com|1I|jIljl|iIlI|ljIjii|Ii|li1jlI
eHpT for
techspot.com,
GoLP@V0sYz$@%9ufMEOZI
.com|1I|jIljl|iIlI|ljIjii|Ii|li1jlI
oGe for
google.com)
3. Creating unique passwords using certain easy-to-memorize and easy-to-lookup methods (For example, google.com is consist of 7, 15, 15, 7, 12, 5, ., 3, 15, 13th alphabet, hence using the last word on page 7, 15, 15, 7, 12, 5, ., 3, 15 and page 13 of the book Harry Potter and the Prisoner of Azkaban as your password), or using randomly created unique passwords and write these things down in your diary
All of above have significant disadvantage compare to a decent password manager.
For method #1, there are two major problems. One of them is a leakage of high tier passwords, since big players are not immuned to those password leaks. The other problem is you can't remember everything. For example, how many accounts have you accumulated over the last few decades? Is it 397 or 401? If you can't answer the question correctly to the last digit, you don't know your password distribution. In other words, you can't figure out which has been compromised or not, even if you have the real time knowledge of every leaks in the globe. Also, even if you do remember everything, it's definitely no easy feat to change the passwords of hundreds or thousands of accounts.
In case of method #2, you need to develop a strong salting methodology, because otherwise your salting is useless. For example, if an attacker knows you've used
TcSoP@V0sYz$@%9ufMEOZI
.com|1I|jIljl|iIlI|ljIjii|Ii|li1jlI
eHpT for
techspot.com, it's no hard to guess your salting method, hence all of your accounts become vulnerable.
The problem is that it's very difficult to develop a strong salting methodology that works all the time, due to all kind of password restrictions enforced on each websites. For example, some websites only accept 12 letters passwords. Which means, if you're using above mentioned salting mechanism, you have all salt no password for abcdefgh.com, and you have broken salt for abcdefghi.com.
For #3, most password creation methods (especially if it's an easy-to-memorize and easy-to-lookup method) usually give you a lot weaker, low entropy passwords compare to what random password generator can offer, and plaintext password written in your diary (or encrypted in your way) is usually a lot weaker than encrypted password manager database stored in offline devices (anyone who lives in your house probably can easily hack into your accounts using your plaintext diary, but cracking the encrypted database is no easy task even for the FBI with a search warrant).
Of course it's true that those password managers are anothher weak point which you can lose many accounts and passwords at once, but like you've said, you can't lose accounts which you haven't registered to that password manager.
Just adding minor accounts to the weaker, cloud-based, easy-to-access password managers and add serious accounts to the stronger, offline, hard-to-access password managers. In this way, you can benefit increased security as well as convenience while minimizing the risk.