form follows function. i understand that spatts trains auxillery muscles in the same plane of motion that is used for the core lift....
NEEDSIZE posted a nice abroutine, with LOTS o static / stabillizing stress, wich i think will transfer especially nicely to squats and deads....
He does partial situps on an incline bench, meaning he lowers his body veeery slowly to halfway and keep that position for 5 seconds, flexing hard, using static contraction after which he slowly moves his torso forward to a full isokenetic contraction
I think this way the abs works hardest when in a semi-upright position, which is also the case with squats
Also worthwhile for improving squat i think are the leg raises as it is a concert of pevlis stabillisation and psoas leg muscles etc.. which are heavioy taxed during squats, especially if you use good form and try to not to lean froward when doing squats......
Offcoarse for cosmetic bodybuilding purposes you must incluse crunches or crunch machine, but i think the benfits do not tranfer as much to squats as the other twoo exercises