The concern is over how much the person actually is "acting the part of a character" rather than being influenced (knowingly or unknowingly) by the mechanics of the game.
Is this the nub of it then? If so then it really depends on how well one grasps the rules as applied to a given situation. With a thorough understanding of the rules of a given game, a player will only have to step outside his character briefly to resolve actions by dice throwing. He'll not be calculating and thus not be using the logical part of his brain to the detriment of his creative side (to be slightly crude about it). This is like how chess grandmasters operate most of the time - they're playing the game in a different way to folk like me who think things though all the time when playing chess.
In any role playing game though players will step outside character at times, it's the difference between a game and a play, in a game, one cannot be immersed in character all the time because some metagaming stuff will always intrude - like dice rolling. A tactical game like 3.5 or 4th ed. can be used to ease metagaming intrusion - by using the figures as a visual aid to clarity, or it can turn the game into something more akin to a skirmish wargame as folk pore over their 'moves'.
But a non-tactical game is still prone to metagaming or powergaming should a participant wish. The player can still optimise his character's actions for the most efficient result.
And just sometimes, metagaming knowledge can add to the roleplay experience. Example:
A friend of mine was playing an aristocrat who was attacked by a beggar. the beggar had just killed an npc with the first shot from a conceled derringer and one shot left. The pc won the ensuing initiative. The GM fully expected him to close. Instead my friend applied his knowledge of the rules. Were he to close, he'd get shot at point blank, not good. So instead he had his gentleman coolly stand his ground (the character had a 'Cool Under Fire' trait so this was entirely appropriate), watching for the man's finger to twitch, so he could preempt the shot and get a dodge in, leaving the enemy with an empty gun. This worked a treat, both from a mtagaming point of view and also because it gave a lovely roleplaying moment as Sir Arthur calmly stood his ground, ready to drop his shoulder and twist his body away from the shot the instant he saw the beggar's trembling fingers twitch. He then drew his sword cane and opened the man's groin after the shot was dodged and the wretch closed with a knife.
He was thinking tactically
and in character, and using a system that is generally regarded as 'cinematic' without miniatures but it would have worked just as well if there had been miniatures and a tactical grid.