Abandoned, or not in use - does it really matter which? Urban exploring means different things to different people; to me it's about seeing places you wouldn't normally get to see. Sometimes that means venturing into places that wouldn't be considered to be abandoned - maybe out of use but guarded like this place, maybe closed but still looked after, or maybe closed parts of an otherwise still operating site. Where should the line be drawn? I think that's down to personal preference really.
It's an interesting debate though, and it got me thinking about why people go to certain places. As an example, why do people explore closed pubs? If you want to see what it's like in a pub, go to a pub! They let almost anyone in, and you can have a nice pint if it's open. Maybe just to see if they can? For the excitement of going somewhere that you're not supposed to go? So in that case, why should these principals only be applied to places that are considered abandoned, as exactly the same can also be applied to anywhere that isn't abandoned. It just adds another level to the "see if we can" thing.