MOI is great if you know what you do. For example if you have concept art or sketches or blueprints. But if you make concepts and experiment with forms like Bulgarod do it is better to use poly, becouse speed is much faster when you find diffrent forms and shapes, you can move parts, rotate, scale, reshape in seconds. And this is only if we are talking about hard surface (Like bulgarov did in "10 Days of Mechs" ) but if we will talk abot organic surfaces like car's body e.t.c. MOI completely loses. So my opinion is that for concepting like Bulgarov do is better polygons, but for precis models for manufactoring - MOI.
I prefer to concept in Poly but there is some situation when some details is better to make in MOI for example text or some array stuff.