I'd like to refute this most strongly!!
Converting a model takes careful planning in order to not balls it up - you might start with a couple of bits which go together just so, but after that the quality of the conversion is down to time and skill.
To start with an abstracted part and create a model around it is a very time-consuming process; the opposite is not generally true as
GW and
FW have gone to great lengths over the years to create 'bits' for easy 'customisation' (the building options for a land raider for example) but that is not converting
imo.
The true skill and quality of the build of a model is in how the 'realism' holds true when observed closely - abscence of mold lines or joins, proper alignment of parts like doors, tracks and hatches, custom scaled equipment or weaponry, accurate limb geometry - these all show off the skill of the build.
My M.U.L.E took a week and a day to build and about the same to paint and i think the build is definetly better than the paintjob!