While I'm not as passionate about this as some were, I hope it becomes commonplace someday so that people can play against friends on a different platform in most games.
And perhaps most of all, this would be a boon for all the lesser known releases. While even today one can go online in something like Call of Duty 2 on the 360 and find hundreds of people at any time of the day, most non-AAA games never build the critical core needed to sustain online multiplayer past the initial release.
Heck, many non-AAA releases never even get off the ground. Doesn't take more than 3 or 4 times of trying to get into a multiplayer match and failing before someone gives up on even trying. Cross-play would help such games significantly.
It looks like this is going to be on a case-by-case basis. I wouldn't be surprised if Sony wants to limit this to games with minimal microtransactions (so they get the money for items purchased on their store, rather than letting PS4 players buy items on, say, the Microsoft Xbox store and then use them on the PS4 so Sony gets nothing) or allows this after a certain grace period, like 6 months to a year.
I mean, its better than nothing but...we'll see what this really means past Fortnite. They've already announced that Fallout 76 is definitely not going to be cross-play.