[@Dirty Dan] Hillan wasn't saying Mokuton is bad (in fact I think he likes it) but more so that Mokuton isn't a KG related to the Senju - there is no evidence of that. In fact evidence points out that only Hashirama had mokuton and it was "his" and not the Senju's release. 

In short: we are allowing it but keep in mind that simply being a Senju doesn't give you mokuton automatically.