Yeah, I would never just "drop in" on a friend/relative unannounced, unless it was some kind of emergency. I HATE when people do it to me. My house is almost obsessively clean all the time, so that's not really it. It's more of, I have a lot that I need to get done, and very little time in which to do it, and so I need to schedule my social time just like I would schedule anything else. Having someone just drop in and hang around - while I'm glad they feel comfortable enough to do that - really stresses me out, because it's hard to put out of my mind all the thoughts of "oh, I told that client I'd call him right back - now what do I do?!" When the visit is a scheduled one, I can make all those phone calls BEFORE the person visits, and then just enjoy the visit.
The only unannounced visitors I like to see are the UPS guy and the FedEx guy. Mostly because they're in as much of a hurry to leave as I am for them to leave. LOL When the cute little Mormon missionaries in our neighborhood stop by, I give them a caffeine-free coke, and send them on their way, threatening to tell them all about women's rights if they return (hey, turnabout is fair play...). I just don't answer when the Jehovah's Witnesses come knocking. And on the off-chance I actually open the door, I tell them (depending upon my mood) that we're Atheist, we're Catholic, or we're Scientologists. That sends them scurrying off the porch in a hurry.
(Am I totally anti-social? I just re-read that, and I think I might be turning into a grumpy old lady...)