Uhm? Just because they don't show up, it doesn't mean they are not there?.
I usually try to keep all quads and triangles, but either way, when you export, you usually convert it to mesh first or else the game engine itself will convert it to triangles anyway, so I don't see the problem.
And as I was saying first, just because they don't show up, it doesn't mean they are not there. If you see the polycount in the pic, it says 68 polygons. That's actual triangles and that's the data that shows up in the 3dsmax viewport info display, so it was actually counting the non-visible triangles.
That doesn't mean I usually go around leaving n-gons. In fact I barely do that, but for a plain surface like this, any non-defined triangle won't break the mesh either way, so I don't see what's the deal. Any of my other models don't have n-gons as far as I know, and those WOULD in-deed have issues if any n-gons were left around, because the triangulation could mess up with the actual shape of the object.
(of course, if I'm going to import it in any other software like ZBrush and stuff like that, I keep it all squares and triangles just where needed)
Happy now? :P
EDIT 2 - This doesn't mean I don't appreciate the critics, obviously. Thanks for pointing it out either way and I hope I can get comments on more content from now on too!