Conventional wisdom is that guards are a dime a dozen and big men that can actually move are a rarer commodity. Unfortunately rare doesn't translate to valuable very often in the PF and C slots. You still need the Gs to break down the defense and get the bigs their touches in their comparatively small comfort zones.
Every great big has had a strong point, SG, or SF to make them shine. Stockton/Malone, Shaq/Penny/Kobe, Duncan, Olajuwon, Moses Malone, and yes especially Kareem. Ewing didn't have much backcourt support compared to these guys so they were always bridesmaids in the East to the likes of Chicago and even Iniana. Yet we see teams go all the way with Centers that are purely role type players.
The problem with guards is that their positions require more adjustment from amateur ball to pro ball.