To be fair, there are a lot of script kiddies in the world.
Simply using the browsers built-in password saving capability with random passwords is still better than nothing (Chrome can even generate random passwords for you). Changing the password for one compromised site is better than having to change it for every site because of password reuse.
In fact, I'd go further and say that you can do this with your login name. So for example:
myemail+by@gmail.com for eBaY
This also helps mitigate those attacks where the attacker actually contacts support and socially engineers them into giving all your info and even stealing your account:
https://medium.com/@espringe/amazon-s-customer-service-backd...
If you are hosting with AWS you should really consider doing that http://www.techinsider.io/hacker-social-engineer-2016-2
Plus, from a practical standpoint, what do you do when a site forces you to change your password because they fear their password db has been compromised (or has asinine rules about your password being too long, or make you rotate your password every 90 days, or they change their domain name 5 times in 3 years)?
1) If your password is cracked, a hacker seeing it might deduce your algorithm and then deduce that your Amazon password would be 'myemail+mo', etc. Now your scheme is out the window.
2) Even assuming your password is salted and hashed securely such that it can't be decrypted, Ebay would probably reset it as a precaution. So now you've got to change this password. Maybe you decide to do the 3rd and 5th letters of the domain name instead. So now your scheme is 2nd-and-5th for all but Ebay, and you've got to remember that the Ebay one is unique. For now. As more leaks or resets occur down the line, however, you'll have more exceptions, to the point that you'll be forced to maintain a list of what KIND of passwords each site uses. In which case, why not just use a password manager?
You just have to turn off any automatic / integration features. I do. Unlock with master password for every use. Really, how often is that, since most sites keep you logged in? Just a couple times a day for me.
(Though I don't use much more than those, on my phone. I do have a laptop ...)