depends on style and skill I think.
Doing a base level without shading or anything does allow it to have a more 'natural' shading coming from whatever the current light source is, but due to that fact that figures are scaled down, you can't really get a natural look like that. They would look like.. well, much smaller figures.
There is also a degree of 'I've got the base colours looking good to me, do I really wanna risk screwing it up by trying to go into the next level of details'. There are some figures where everything seamed to be going okay for me but then I went a little too far.. and once you have done that, you might as well just give up right away.
some 28mm figures aren't into exaggeration of design and these never look right to me... but fair enough, each to their own. mostly there are digital created sculpts because trying to do the detail at the scale is.. tricky or they are big companies that can use tooling down machines (like done in the action figure market)
