[@Reflection]
Not all logias have elemental weaknesses like smokers, and unlike the ope-ope no mi, logia fruits offer their users a direct defense without any limit.  Where as the ope-ope doesn't grant the user any such think. 
Plus there's the stamina issue, which has already been pointed out.
As for the soul swap i don't think that'll be an issue as my character won't really be knowledgeable in the medical field