Being slightly facetious here, but one reason not to grade skip is, if it goes too well, you might need to do another one.
My daughter skipped 2nd and was in such a wonderful school, that by the end of 5th, she had easily worked well ahead of her grade peers. She ended up skipping 6th (the first year of middle school here) and into 7th. She'd probably be a candidate to do it again before graduation, but middle school subject acceleration looks like it will feed nicely into high school where there are a ton of different (and AP) class options to keep her occupied.
I would say sports have been a little bit of an issue. Even being a little big for her age leaves her small for her grade, which can be a big deal on swim team. Socially, she is a little out of step, not being into make-up and who is dating whom. But, all in all, I'd be in favor of her doing the accelerations again.
