The problem with loops is that you are limited in your creativity. You are reliant on the loop's arrangement, not your own.
However...it still takes skill to mix loops creatively. Bad looping will probably sound like rojak with everything out of place. Done creatively, I'm pretty sure it will sound good.
I have some loops (one of my favorites is Spectrasonics' Metamorphosis), but I hardly use them. I would rather create my own. But in some situations, I would still use them.
Do pros use them? Yes, I'm sure. How do I know? Because I recognise some of the loops!!! I know which library they came from. In particular TV series - I can't stand it when they keep using loop scoring libraries (and many times, the same ones). How do I know? Because I have the same libraries!!! For 99% of the general population, it wouldn't matter since they wouldn't know. But if you are using samples, you can probably identify them if you've keep track of libraries out in the market. That's one of the problems with loops.