People have been describing two different situations: holding a site briefly and reserving a site that you do not even stay in one or more nights.  The later is never acceptable, no matter what you leave at the site.

Leaving just a single thing, even a pack, is iffy because someone could think it was forgotten.  But leaving behind a bunch of stuff (or a pack where gear has been removed to some extent) clearly shows someone is occupying the site.  Setting up a tent is not required.  I can even see a scenario where someone going out for a day trip but returning to the same campsite might take down their tent because of concerns of what weather might do if the tent is left up (maybe the tent is in a location where it is hard to tie it down and the site is quite exposed to wind).

Azalea makes an important distinction.  To drop some of my gear at a site while I go prospecting for something better is very different from dropping off my gear with a clear intention to stay while I take advantage of the day and go off and fish or check out the lake.  The problem is that for the person coming upon the gear it is impossible to tell the difference.  Personally I would choose to believe the person is doing the latter rather than the former but to be quite honest its never actually happened to me and these days I'm tending to do my tripping in more secluded regions where it is most probably a non-issue anyway.  Philosophically speaking the answer is, as has already been pointed out, adherence to the Golden Rule.

Wouldn't do it, and have never seen it done.  Take the time to set up camp, it just makes sense.  No one likes to fish more than me, and I would always set up camp before going fishing.

Twice I've had to share campsites with trippers who weren't on the ball all that well.  They got into areas at dusk where the only campsite around was occupied by us.  Both instances were in Lake Superior PP, not the Q.  Both went well, no problem.  Not ideal, but you roll with it.

Mark