I'm no expert, but I don't think leaving the wound open intentionally is very common. Sometimes a wound will open becuase the stiches break, and then they will leave it open. I wondered why they just can't re-stich it, and from what I gather, it's something to do with infection. Unless they re-prep for surgery, they could not create a sterile environment.
My perianal stiches opened up in the last day or two of my hospital stay. They just told me to pack it and sent me on my way. Not fun - incision was huge and took months to close. I'm 11 months post surgery and still have an annoying little hole that just won't close. I visit a very nice local colorectal surgeon every few weeks to check it. It does not botter me at all. (it just likes to drain a little now and then)