Just because it is common knowledge does not mean that is is correct. There is a lot of "common knowledge" around here that is dead wrong. And if not fully cured it may fail, it may not, lot of factors besides it being fully cured.
That all said, I think it is stupid to use GE I or II for anything aquarium building related. I completely agree on your statement, why chance it with cheap and crappy silicone? Buy the good stuff and do it right the first time. RTV 103 or 108 will hold properly, there is no risk of toxins and it really isn't that much more expensive. When you have hundreds of dollars of fish what is another $5/tube of silicone?