That doesn't seem like a sufficient degree of caution, considering how McGonagall behaves with respect to other potentially dangerous Transfigurations. She doesn't say to only Transfigure things briefly into liquids because of evaporation, she says not to do it at all.
And she especially wouldn't do a potentially dangerous Transfiguration as the introduction to a Transfiguration safety lecture without making a big deal out of all the precautions she had to take to do it. In fact, she specifically says that the students could guess the direction of the Transfiguration based on safety principles, and based on safety principles, it should have been pig to desk, not desk to pig.
Well, if the pig "is" actually a desk, then perhaps it would only look like it's breathing. It would be a magically animated simulation of a pig crafted from the matter of the desk. Even if it's sucking air in and exhaling it back out, it wouldn't be actually metabolizing oxygen, at least not any more than its "actual" desk-self does. Since Transfiguration is based on Platonic metaphysics, the desk would be a pig "in substance" but still a desk "in essence" (which is why it turns back to a desk when the spell wear...
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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