In my Experience a Battlemat has one major Advantage. It helps clarify what the GM thinks the situation is like. Whenever verbal Descriptions are given, a group of six players will come up with six different visualisations in their respective minds.
So i agree that for small and quick scenes, setting it all up on a tac is not worth the effort. But the GM has to be specific and should assure that his players have all the information "before" the make their rolls. Its a bit frustrating if i think of a real cool move, just to be told something like :"Yeah! That would be a great Trick. Sadly the 3 Bad-guys in your path kill you while trying..." Though this surely can to some degree be very satisfactory for the GM.
If there is an error on my part, as GM, of communication, then I certainly rewind the clock a bit a replay that section of combat, within reason. Or if a player so misconstrues the situation and attempts something really out of character and simply strangely dumb, I'll stop the game and go "really?!?!?" Sometimes, the action is totally justified and unexpected because of roleplaying... a sudden flash of insight and YeS! They wanna try it! . Sometimes, the player simply didn't hear some vital tactical clue. And that can make a lot of differnce in reaction.
But trust has been built up among the players and gms of my groups. I know when someone isn't quite getting my drift on the situation... and they know that I'm not out to screw them, that dramatic editing can always happen with a Benny toss for a coolness factor, and generally ask good questions to clarifty the situation.