Not sure if you intended it as such, but this is a valid issue.
I also have a completely sincere question that came to me as I was reading the "whatever works for the situation" proposal. If you're using TotM for the battles that don't require much precision or thought and grids for more complex ones, then presumably these TotM battles are ones that the DM is sure the players are going to win. So, my question is, why have such battles that amount to minor speed bumps at all? Isn't it just a way of drawing out the inevitable "Orcs charge you from around the corner, but you kill them easily. What do you do now?" If all that stands between "orcs attack" and "you kill them" is a few dice rolls, while other battles require significantly more time and effort, why have the simpler ones at all? Why not make every combat encounter equally complex (if not equally difficult)?
Or is the suggestion instead: All combat should be difficult, but some is straightforward (e.g. orcs charge you from the other end of a long hallway) while some is complex (necromancer down below, skeleton archers swinging from the chandeliers up top, difficult terrain + environmental hazards around)?