I wouldn't consider it crack... I mean nothing is crack if it's written well and there is the necessary character development. I've read a perfectly believable and justifiable IchiruxKaname.
It's more of a matter of what's unlikely in canon... A truly hard to write, if ever written would likely be crack, would be YagarixAido/AidoxYagari. To a certain extent I think people could make YagarixKaname/KanamexYagari work.