BIOCHEMISTRY LESSON TIME!!!!!
All normal sugars exist in two forms, linear and cyclic. A linear "sugar" is defined as a molecule with a chain of at least 3 carbons and at least 2 of those carbons has an -OH functionality on it. The -OH is called a hydroxyl group or alcohol group. The carbon without the -OH has a carbonyl group, which is a C=O where = represents a double bond (bond with 4 electrons in it). Now the oxygen atom in the OH has a lone pair of electrons capable of reacting with the carbonyl in something known as a nucleophilic attack. This cause the linear chain to form a closed ring. The most common sugars have 6 carbons (glucose) or 5 carbons (fructose). Glucose can form a 6 membered ring, known as pyranoglucose, or a five membered ring, known as furanoglucose.
NOW, the closed rings can react with each other and form bonds between rings, known as dimers. Dimerized glucose is known as succrose. When you ingest succrose your body breaks the glycosidic bonds between the rings and send the cyclic glucose into the krebs cycle where it is converted to energy. The cyclicity of the ring is requisite for energy production.
SO if you "reduce" the carbonyl functionality (convert the C=O to a C-OH) then you can not cyclize the ring and as a result the sugar can not enter the krebs cycle. Therefore a linear sugar can not contribute to insulin spikes or the glycemic index. That being said, your body is very good at finding energy and has enzymes which will oxidize a hydroxyl to a carbonyl, making the sugar alcohol an active sugar. Although these enzymes exist they are inefficient.
So bottom line, sugar alcohols are NOT as bad as "regular" sugar but they are not truly 0 calorie. Their contribution is, however, minimal.