That depends on what you're doing. A separate login is useful for stuff like cross-platform support. But also, if your game is some sort of MMO or live game you may need some sort of account management if you want to sell it stand-alone.

These days some people just plug into the storefront's login system, but I also don't begrudge the ones that don't, mildly inconvenient as it can be. Especially not the ones that own their own storefront and are only on Steam because you can't not be on Steam.

Which, again, I find to be a bad thing.