When building Dapps you don't need to login. For example, in the CryptoKitties Dapp if you'd like to sell a cat that happens through the blockchain and there's no need to log in to the CryptoKitties own servers. You just need your private key and you can send the transaction.
But certain functionality isn't on the network and does run through their own servers, so here they use MetaMask to login and this enables you to set a username and email (to receive notifications from them).
It would definitely be nice to have an out the box solution for login using MetaMask as more and more Dapps start to be built.
Article: Never Use Passwords Again with Ethereum and Metamask
When building Dapps you don't need to login. For example, in the CryptoKitties Dapp if you'd like to sell a cat that happens through the blockchain and there's no need to log in to the CryptoKitties own servers. You just need your private key and you can send the transaction. But certain functionality isn't on the network and does run through their own servers, so here they use MetaMask to login and this enables you to set a username and email (to receive notifications from them).
It would definitely be nice to have an out the box solution for login using MetaMask as more and more Dapps start to be built.