“The mystery to me is why Lee — ever the brilliant tactician — dismissed them.”
I think your answer could be found in the book “Lee’s Real Plan at Gettysburg” by Troy Harman. Harman is a park ranger at Gettysburg and has done a lot of historical research.
I read the book a few years ago and it really did make a lot of sense. Lee was really not trying to punch at the center of the Union line ion the third day (Pickett’s Charge) but was angling northwards towards Cemetery Hill, his objective thru the whole battle.
Chances are, you can find this book at your local library. In any case, her’s a link to it on Amazon.com:
http://www.amazon.com/Lees-Real-Plan-at-Gettysburg/dp/0811700542
Not to completely derail the thread, but even if that was Lee’s true plan, ignoring the Round Tops would have given the Union an opportunity to flank an attack on Cemetery Hill. If the crippling Union artillery had been ensconced there, no attack on the center would have stood a chance.
Armchair general here ...