I'm really glad you asked me that, Kev, and I'll tell you.
The difference between a band covering another band's song and someone sampling someone else's music is that the band isn't usually playing in front of 25,000 people, playing music that someone else wrote. At least not a band of any merit. If you're in a cover band, then you're a hack, sure. But you're not pretending to be something you're not, either. You're playing clubs and making money like it's a job...it's not your creativity. You're doing it because it's just fun and it's an easy way to make some cash while simultaneously getting your band recognized so you may perform your own originals eventually. You're still a bit of a hack, but that's just how it is. It doesn't necessarily mean it's a bad thing.
Now, someone making their entire career off of someone else's work, i.e., just about every single popular hip hop artist since Run DMC and many techno/trance DJs fall into this category. Some of it's absolute dogshit and some of it is a work of genius, but the fact remains that all they've done is rearrange a piece in a way that would be accessible to either the mainstream or the already very strong following they've gathered. They haven't created anything, all they've done is add technology to it, and the saddest part of all of this? Most of their fans won't even know that it's not even the artist's own work. I'm sure everyone has heard Adagio, so this isn't the case here, but you see what I mean.
That said, I love cover bands. I love seeing people recreate something and even improving upon it, but it has its place, and that place isn't in front of 25,000 people.