As others said, it depends. There are some songs that definitely call for it, and there are some where it just seems like they couldn’t think of a better way to resolve it. The thing is, not every song can have a clever ending or can easily be resolved, and it’s an easy way to handle those sorts of situations.
Personally, I don’t really think to much of it unless it stands out. I remember listening to an album several years ago that was otherwise quite brilliant, but it struck me that every single song on the album ended with a fade out, and while I still loved the album, it’s not the sort of thing I should have noticed and I’d count it as a negative. It’s sort of like when I saw Battlefield: Earth and became aware of the cuts because there were only maybe 2 or 3 diffrent ones used in the film. Song endings, like cuts, shouldn’t stand out for their similarity, unless it’s deliberate to an end.
That all said, I do think it can serve several really cool artistic purposes. One major way I’ve seen it used, and a way I’ve used it in my own work, is as an indication that there’s a continuity, as if to infinity, at the conclusion of the song. That is, it’s precisely that the song doesn’t have a resolution. One good example of this is a song by Nevermore with a rather pessimistic view of the existence and nature of God, the song ends reiterating the the phrase “Don’t set your mind to one side” phasing between the left and right speakers and fading out. I think it illustrates the nature of the discussion and, in the view of the artist, the nature of God himself, and how it doesn’t have any sort of resolution.
For a more emotional example, I’m also reminded of the song Drapery Falls by Opeth. It’s a long song about the confusion inherent in a failed relationship, coming to a realization of that disfunction, and continuing on it the cycle anyway. This is musically illustrated by having an opening riff that sort of has a confusing and falling sense, imprinted with the first lyrics “Please remedy my confusion”. After exploring the concepts and coming to the realization, the song ends up with the lyrics “Waking up to your sound again, and lapse into the ways of misery”. He’s pulled the drapery down, seen the essence of what its there, but returned to the very same state where he started. And thus it ends with the same riff that it started out on, fading away for the cycle to repeat.
So, yeah, it can be lazy, and it usually is, but it can also add another level to the song if done well.