In general, I'm not lyric person, but I think that if you look at this song as a relationship song (probably one about Bono's with ali after his dad died), it's pretty damn coherent. I initially thought it was a nother God song, and it probably is that too, but I'm not well versed in that area, so I don't know.
As others have said, the song was set for release. The only thing unfinished about it is that it probably never made it to mastering, thus the mix that's overly bright in the mids/highs and lacking any meaning ful bass.
As for it fitting on the album, of course it fits. What does AMaaW have to do with the rest of the album? CoBL doesn't carry some overarching theme that the rest of the album has. And if we're talking about how they sound, Vertigo is unique and One Step Closer (yawn) certainly doesn't fit in.
If the theme is Dad's death, Mercy probably fits. If we're talking about all the God Themes, the allusions are certainly there, though whether the character with the vision ("telescope") is ali or God, I don't know.
And finally (I'm sure I've said this before), if it'd been on HTDAAB, it would have been far and away the best song on the album. Then again, that would have been true on virtually any U2 album.
The bottom line is that when you've written and recorded one of the 5 best songs of your 25 year career, you don't cut it from the album.