Are you sure that you aren't mixing up the differences between Silicone I and Silicone II with the differences between different varieties of Silicone II that you can buy?
At my local Lowes and Home Depot you can buy two types of GE Silicone II - 'for door and window' and 'for bath and kitchen'. The bath and kitchen variety contains mildew inhibitor, the door and window variety doesn't. Otherwise they are exactly the same stuff, that is they would rate the same on the adhesion scale. Now, the tricky part is that these types of silicone are not typically sitting side by side on the shelf for an easy comparison, but are in the door and window and bath and kitchen sections of the store, respectively.
GE Silicone II for door and window (ie without mildew inhibitor) is what everybody recommends for aquarium use, and also the same stuff as what is sold under the Allglass brand name. My personal experiences with GE Silicone II for door and window are excellent. I used about 12 cartridges of the stuff to attach a rock background to the backwall of my 240G with no ill effects, the tank has been running for more than 2 years now (for details click here
. I have a 29G with a similar background using the same silicone that has been running for more than 3 years, also with no ill effects.
A scientific mind might wonder whether or not the toxins that kill mildew in the bath and kitchen variety of GE Silicone II could potentially harm fish. Well, I have no scientific data to prove they would, but since I can buy the same silicone at the same price without mildew inhibitor and play it safe, I don't see myself ever using my expensive Tang cichlids as guinea pigs for the long term effects of low doses of arsenic on fish.
Until I read you post I had never heard or thought about the use of Silicone I in the aquarium, but if it's lower on the adhesion scale as you say, maybe that explains why it's not used.