Suppose it was an overcast day, you shot the exterior of your listing, and you're now ready to go back to the digital darkroom to do some sky-replacement magic - that's when you realize that you don't have a replacement sky. Instead of fast-forwarding to that day, do yourself a favor and start shooting skies now.
It's so easy - just be disciplined about shooting clear skies when you see them. Personally, I just open the back door of my house when it's nice outside and snap a few, download 'em and file 'em away. If you have multiple cameras with variable resolutions, do yourself a favor and shoot with the highest-res camera you have. I'm not a pixel-peeper by any stretch of the imagination, I'm just trying to save you a headache. If you have a folder full of 6 MP skies and you're shooting exteriors with a 12 MP camera, things may not add up with you put the pieces together later. Sure, you can resize the layer giving you the trouble, but just save yourself the time and shoot with the best you currently have.