Sure, extensions are great, but there are a lot of users that aren't going to go through the steps to install a browser extension to use a website. Also, an extension is good for my laptop but it doesn't help me out much on mobile. 

With this setup, you are able to have the convenience of a custodial solution, but, you aren't able to get rugged if they act dishonestly. Now you can effectively rotate your keys without comprimising your npub