You can find an article on Lawless' own website where he says he is cool being on stage and he is cool not being on stage. He doesn't "prefer" being off stage, he's just cool being an employee of U2 and will do what they ask him to do.
It's all about the sound. If you can produce the sound, then produce it. If you are resisting producing certain sounds because you only want the romanticized bullshit notion of "4 guys" on the stage, that's just as bad and archaic as the purists that are so adamantly against anything canned being played at all.
Take Gene Simmons...he's supposedly a purist about sounds not being canned and yet he needs pyrotechnics, makeup and other assorted theatrics to distract from their vanilla sound. It needs to be a "rock show" because there is little else. Some artists think their songs are good enough on their own. At one time, U2 used to believe that. Now, they're much more like Kiss. Hell, I'm cool with giving the fans a show, especially if you want to charge them a relatively absurd amount of money for tickets. But once you enter this realm, you've abandoned this purity argument in all forms...whether you're Gene Simmons or U2 keeping Lawless under the stage because of a romanticized notion of purity...or simply ego or both.
Musicians should record live, not because "it's real, man" or any of that stuff but because 1 - it sounds better and 2 - it makes you a better musician. That is - if you're concerned at all with being a better musician. Pop music, canned stuff, is fleeting. It doesn't have much shelf life for a reason. I am just as bored with most of that shit as I am with the stale stale stale blues-based rock that bands have shat out for upwards of 50 years. Take shit like the Black Keys and burn them. Please. I'd rather hear canned pop stuff like Charli XCX all day every day than them (and even she uses a live band for live performances). I don't care if there is a backing track. I love loops, I love canned stuff and have used plenty of it in my own music. Some of the best U2 music has that stuff strewn all around it. It's all about the sound. Make a cool sound - who cares how you get there? Who hasn't done this stuff? I mean, besides purist fossils that need to step in the 21st century. But it's not about purity...it's about what sounds better.
The important reason Radiohead uses the dude from Portishead as a 2nd drummer is so that the additional percussion is LIVE...that this is also some ancient ethical "no back track" purity stance is actually entirely beside the point. The point is to add a human element, even the beauty of human error and magical accidents, to the feel of the rhythm as opposed to canned beats which are entirely up and down and don't move.
This is why it should be played live - it sounds better. Imperfection is wonderful. And certainly if U2 is gonna use an additional musician they ought to be bring him out from behind or under the stage. Again, it's just as stupid as what Gene Simmons is saying. U2 want to feign 'purity' when their ego is involved ("another guy on stage besides the 4 of us? No way!") but when it comes to those other codified standards, they shed them with ease. Reason # 4,815,162,342 why the U2 ego is run amok to the point where they aren't worth being defended against just about anything anymore.
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