I would think that JB has a less expansion property than old polyester fillers that we are forced to use and considering its superior bonding to metal property, I have no quibbles with using JB as a filler. my experience with bondo is it is less forgiving in the sun and absorbs moisture more which leads to separation more.As far as being professional goes, Whats your point? Its all about what works best.I use JB weld for small spot putty smoothing over rusted pitting spots.It will not swell in sun and is solid as a rock. much better than bondo! I guess some people are set in thier old stubborn ways but my personal advice to beginners looking for advice here would be go ahead and use JB for small imperfections under 1/16 thick for just leveling surface. Iused JB weld to rebuild lip groove on dryer drum that turns on wheels and gets warm. the dryer drum lasted for years.
There is no replacement for common sense.