FWIW, I always think of the multiple single-year skips as a way for the child to learn two years worth of material in one. Skipping two years at once is just about being ahead. But remember that older doesn't mean faster. The child will still be getting material at the same pace as everyone else. I think that's the reason multiple grade-skips are recommended.

I guess that applies to multiple 2+ year grade-skips, too. The child just learns 3 or more years of material in one year...

Kriston