SAJ, I'll answer your question with another question. How much experience does a coach need to have to be appointed to an NBL team? I'd say Clarke's experience thus far is very good, as you say he's not coached professionals before but your coach in all likelihood is going to be either:
a) Previous NBL coach being recycled (Joyce, Stacker etc)
b) American college coach
c) Australian coach with no NBL head coaching experience looking to take the next step
There are all sorts of problems and reservations with appointing Joyce, as has already been discussed to death, and one could argue that Stacker hasn't seen much recent NBL success either so none of those appointments would have been particularly inspirational.
An American college coach would have the same issue of never coaching professionals before, but would have the additional disadvantage of being unfamiliar with Australia's young talent.
This leaves an unproven Australian as the final alternative, yes, it's a risk appointing someone new, but in the absence of Goorjian or Phil Jackson beating a path to our door I think Clarke is the best available. He also comes with a tremendous upside of having some recruiting clout when it comes to yong Aussies returning home.