Neither the statement that it "loses" calibration frequently nor the claim that you need to point it back at the sensor bar to regain calibration is true.
Motion Plus, like every other motion control system, is attempting to define vectors in three-dimensional space. Sometimes it only needs coordinates relative to its own neutral state, sometimes it needs coordinates defined by some external point or direction. An example of the former is the tightrope "balancing", which appears to only require waving back and forth and down (to "jangle"). These are all relative to the controller itself- right is the right side of the controller, left is the left side, down is toward the bottom of the controller. This works every time I attempt it; no re-calibration is necessary.
For coordinate systems relative to some point or direction in the world the controller does different things depending on the task being asked of it. One "reference" vector it locates is "down": this is accomplished when you place the controller on a flat surface each time the game starts up or you allow the controller to turn off and then back on. The accelerometers detect the pull of gravity and set that direction as "down", and it appears to retain that calibration over long play sessions. This is used during the sword fighting (to distinguish between vertical, horizontal, and diagonal slashes) and most obviously during the "Skyward Strike" move. I've done this move after three+ hour play sessions without needing to re-calibrate; that's pretty impressive (I honestly wonder if it isn't stealthily re-calibrating when the controller is held still, without player interaction, to maintain the "down" vector).
A second "reference" in the environment is the location of your television.
This is calibrated at the start of each game session by pointing at the screen and centering a pointer, aided by the sensor bar. I presume this is the part that people are complaining is getting "out of calibration". Except... that's not precisely what's happening. The location of the television defined during the "Point at the screen" step is not, as far as I can tell, used for more than the first couple startup screens (picking your save file). After that point, every single time you use any weapon or in-game element that requires "pointing" the game assumes the direction you're holding the Remote when you start the action is center. In other words, it "recalibrates" (really, re-defines center) every single time you look around with C, every time you aim with a weapon, every time you
, and so on. As far as I can tell it never uses the sensor bar again from that initial "calibration" forward.
I suspect people like Gigglepoo think this is "losing calibration" because, on occasion, you may end up aiming at a spot that clearly isn't the center of the TV- down and to the right or whatever- as though it's straight at the screen. This happens precisely because you were pointing that way when you started the aiming action. There's also the possibility that, if you whip the Remote around particularly violently, you may max out one or more the sensors and end up pointing incorrectly; these are the reasons the "Press down to Center" button exists.
Why do they redefine the aiming "Center" every time you start an aiming action, rather than just depending on the initial location of the television? I suspect it actually helps people playing. You don't have to point at the TV at all... you can point slightly down and left at all times if you wish. In fact, that will probably happen naturally and unconsciously as you let your hands rest as you play... People have always griped about having to hold your hands correctly to play Wii games, straining wrists and all that... Well, with the Zelda implementation you don't have to rigidly point right at the screen at all times (particularly helpful since you're switching to aiming items so frequently in this one). The game adjusts to however your holding the Remote at a given time, and gives the "Center" option just in case you happened to be pointing uncomfortably off in space when you started one particular action. The other reason to frequently redefine the "center" is that it represents the line drawn from the player to the screen. Unlike gravity, that line is likely to change when the player moves. If you move from one end of your couch to the other that "center" line shifts by a fairly wide arc. By constantly redefining that line the system allows the player to shift and move around with bugging him with a "recalibration" screen.
