I don't think that aspect is a pro of Slack. However, the UI capabilities of Slack are certainly pros.
Devil's advocate: the only way to have a single "account" for all of your projects on IRC is if they all use the same server. Is that not a weird, perverse amalgamation in and of itself? Taken to the extreme, every project that wants to truly control its own destiny ends up using its own IRC server, and you're back to N tabs open somewhere.
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.
Can someone explain to me how having to create a separate account for every project you have a question about is a "better" than anything?