Aside from the obvious intuitive answer that you're just cutting and pasting someone else's work. But hell, at least you [hopefully] paid them for it. If you make your own loops like I used to, then kudos to you for the effort.
That being said, I've got nothing fundamentally against including them in your work as an artist. There's my preface.

So as some of you might know, I've been away from this scene for a little while, in between computer crashes and getting hammered (by higher education this time - alcohol, unfortunately, was only a small factor there)

But now that I'm getting back up to speed here, I'm noticing endless amounts of repetition inside a lot of the top tracks. I.e. it's obvious that the common misconception here is that original music constitutes nothing more than an intuitive arrangement, rearrangement, and repetition of loops. While some of you do a great job with this, others need to rethink their approach a bit.

First off, I'm gonna state the obvious good things about loops.
#1. They prevent you from having to go out and buy, say, a drum set or a guitar, or any number of other pieces of hardware few of us can afford. (Not to mention the time it takes to learn how to play an instrument, or the time it takes to get good at it.)
#2. They save a hell of a lot of time. Face it, engineering your own sounds and FX can be time consuming. And being able to stretch a simple wave file through the length of your song is sure easier than placing any number of sounds yourself. Same with writing your own notes.

Sure as hell is a convincing argument for using loops, ain't it? Truth is, the biggest problems I have with loops come right out of the second element listed above. Let me state it another way:
#2. Engineering your own sounds and FX is an important part of the creative process, and the temptation to fill up a ton of space with a handful of really cool loops should be the 8th Deadly Sin because you're cheating yourself by cheapening your song by leaving little room for appropriate adjustments.

How man times have I been listening to a track I thought had enormous potential, and just when a well executed key change or sharp note would have absolutely made the song, I'm let down by, guess what, another, another, and another repetition of the same damn loop. Whether it was cool the first time I heard it or not, it's lost me by the 8th repetition. Using loops, especially complicated ones including a number of instruments and sounds which can't be modified independently result in this almost every time.

In addition, the fallacy behind the tendency to overuse loops that seems obvious to me is overwhelmingly pervasive. The apparent quality of a cool loop leads artists here to believe that there's no need to alter the clip at all throughout the song, whether for a key change or a shift in your equalizer. Either one of these changes placed appropriately would work wonders. Yet almost no one's doing it.

And philosophically, I believe that there's no beat in a track that can't be made better in some way. Maybe that's why I take so long to release material. I don't know. Sometimes it's something as simple as strumming a different part of the guitar string. Or changing the volume of a snare by a decibel or two on certain beats. Everything should be intentional, and everything in your track should be there to help support the concept behind it. This is something you'll have trouble doing with a loop.

And if anywhere in your mind you're intending to make your track sound "real," remember that real instruments don't play the same every time you use them. A guitar note can be played a million times, and like snowflakes the waves produced are not once repeated precisely. Also, a number of identical repetitions cues any listener to know you're stuff is just mixed, not recorded, i.e fake and not real, whether the loops are of your own creation or not.

Finally, if you're still going to use loops (which, if you spent a bunch of money on some libraries, and you really, really like them, you probably should), remember that if you found a loop you want to base a new track on because it speaks to you, or it's just cool as hell, you're cheating that piece of sound out of it's potential through not pushing it to new levels with your own improvements, or using it in conjunction with a variety of accompaniment that varies at least subtly through your track. Not doing so crosses the line between taking advantage of all that potential, and simply cutting and pasting, rearranging and repeating. Whatever you're trying to say with your music, then, will be lost to what amounts to laziness, intentional or not.