For what it's worth, the DRM on Steam is an absolute charm compared to some of the others out there like UPlay for example. I'd prefer Steam over any of the others any day. Though sure, I'd prefer to not need it at all. But then updates would be a lot more cumbersome than they are with Steam.
Regarding the buying a PS4 version of Saints and being able to play online with those playing it on Steam / PC. Well, I don't own that game, and it is very game specific, but I highly doubt it. I think the only one I've known that happen was Portal but that was because Sony and Steam did some deal and you could sign in to Steam on the PS4. Maybe they've done that again? Otherwise the usual case is that PSN players play online with other PSN players, XBL players Vs other XBL players and Steam Vs Steam. It's not normal for them to mix. Not completely out of the question though.
Regarding the buying a PS4 version of Saints and being able to play online with those playing it on Steam / PC. Well, I don't own that game, and it is very game specific, but I highly doubt it. I think the only one I've known that happen was Portal but that was because Sony and Steam did some deal and you could sign in to Steam on the PS4. Maybe they've done that again? Otherwise the usual case is that PSN players play online with other PSN players, XBL players Vs other XBL players and Steam Vs Steam. It's not normal for them to mix. Not completely out of the question though.