Totally fair. Having to make a new account for every project is definitely not ideal, and I wish Slack had better integration for more seamless sign-up using existing identity providers.

Slack does have a few ways which can make it a little easier to sign-in on mobile, though, such as sending you an e-mail link that opens the app and signs in for you. Likewise, once you setup the desktop client, you don't have to relogin unless your password changes.

It's a small initial overhead -- one that IRC doesn't present if you're going to the same server as you mentioned -- but one that I don't think is quite that bad. I've never been in more than five distinct Slack teams, though, so, maybe this would just explode if we literally tried to replace IRC with Slack for the top FOSS projects.