You're right - the T-pose (or its cousin the A-pose) is usually the default pose we put a model in when a particular pose or animation isn't working. The T-pose exists mostly because artists can see all parts of the model by rotating, moving, and zooming the camera, so they can fix up any odd texturing issues on the model.
The purpose of using the T-pose for the default is because it is instantly and visibly recognizable to just about anyone that it doesn't belong there. We often use things like a checkerboard pattern or bright magenta/cyan colors on a cube to indicate a missing model or texture so that the game doesn't crash when it's missing an asset, but QA can immediately flag it as not working properly or using a placeholder asset. As such, we don't want to hide them - we want them to be as visible as we can so that we can fix them.
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