Plenty of reasons. Shorter means that it's easier to get through the whole thing in one sitting. It also could have better song sequencing. A full show's worth of songs is exactly that: a full show. A live album on the other hand does not imply a full show, nor require one. It's an album like any other and should be treated as such since albums are fundamentally different experiences than full concerts. Changes should be made to the number and order of songs that are included to optimize the experience for the medium. There are many classic live albums that flow better as their album versions than the full shows that may have surfaced later on. Made In Japan, Live/Dead, Genesis Live, At Fillmore East, Live at Leeds, The Last Waltz, all have had extremely extensive repackagings that expanded significantly on what material was included. and while the end result is an anthology release that might be great for the fans and important for rock history, rarely if ever do these supersede the impact and significance of the normal versions of the records. And even more rarely do they function as superior "albums" when compared to their counterparts.
>>Some mixing choice are better than others.
Sure. Mixing is important obviously and poor mixing should be taken into consideration when reviewing an album. But to say that poor mixing results in an album that is completely worthless is really really stretching it.
Besides I highly doubt that a change in guitar tone fundamentally alters the songs so drastically that it renders them unlistenable. The way you describe it seems like this case is probably just minor matter of taste. The band chose a lighter guitar sound to make it more commercial and since the album was relatively successful I would assume that plenty of people liked the more commercial mixing. It hardly seems like a major mixing argument like the extremely different mixes of Raw Power, or whether the mono or stereo mixes of The Beatles records are better.
>>Similar to Under The blood red sky, I have the CD and the DVD mix of both version ripped in my computer. Now can you give me a single good reason why I should listen to the CD version if I think it's inferior to the DVD version?
If the DVD version was just the CD version with different mixing and an expanded tracklist then I could easily see the argument for considering the CD redundant. However the CD *isn't* redundant since it consists of mostly completely different performances. Going by what wikipedia says, the CD gives you six performances of songs that the DVD does not. That's not a ton of material by any means, but it's still something that you don't get on the DVD. So it's worth it for that at the very least.
As soon and you start ripping DVD audio, making your own custom track-orders and stuff like that, you really can't rate and review that modified version as if it were the original since it's not the original and you would be misleading your readers. John, for example, shouldn't grab the DVD audio of Blood Red Sky and review that as the audio track for his review of that album, that would be misrepresenting what is actually up for review. By all means he could still talk about the DVD material if he wanted to as a supplement. But unless he actually did a separate DVD review it shouldn't affect the score.
I should also point out that my arguments here are primarily just for the principle of the thing, rather than a specific defence of this particular record. I have never heard this live album or seen this DVD and I don't really intend to go out of my way to do so anytime soon as I think U2 are an exceedingly mediocre band whether they're in the studio or live.