I haven't posted in a while, but still following this case closely.
My opinion has always been that IF Maddie was not abducted and did die in the room that night, the McCanns' covering-up would make perfect sense.
These people are extremely image-conscious. That's certainly not a bad thing in any way. However, the way I see it, if Maddie had died in a car accident or drowned in the pool during the family vacation, the McCanns would have everyone's sympathy, and deservedly so. What a tragedy, right?
BUT...if Maddie died accidently (which I believe she did) that night, while left alone with her siblings, what kind of sympathy would the McCanns receive? Of course, people would feel bad for them, but there would always be that undercurrent of "Well, they left her alone, what did they expect?" They would never be the victims of a tragedy--there would always be the whispers that it was, after all, their fault. Their social standing within their group (which sounds like it was extremely important to them) would be diminished. They would have lost their daughter AND their reputations.
So, if they could no longer save their daughter, they could still try to save face, and hence, the abduction scheme. They are victims now. Friends and family are showering them with sympathy.
They simply did not count on a public who would not do the same.
IF they are guilty, of course.