On one hand, it's as Kate says: too easy a fallback.
The other hand is slightly subjective. Consider how people smile. It's a continuum, from the passive face through something hinted at, to satisfaction, to maniacal. I suspect that a lot of the objection to smiling is that some people interpret it – by default – as the upper end of that scale; they don't notice the lower end of the scale. And there are also people who feel that others who are permanently a few notches above passive are somehow derranged or hiding something or… (Knowing that life is out to take the piss, I've got a permanent smirk on my face, and as such have been on the receiving end of such opinions.) So, effectively, this is an issue of lack of precision.
I would add that the danger or having your characters smile or grin or even shrug too often is that the *one time* in your book when that smile or grin or shrug actually signifies something important, the reader might not notice it.