monosaccharides can also be alosteric regulators (think of fructose 1,6-bisphosphate in glycolysis) and so can disaccharides (think of lactose in the lac operon). Polysaccharides have a structural role in the cell wall of plants (cellulose) and fungi (poly-N-acetylglucosamine), they are important for cell adhesion (in cell junctions), they are important in the immune system (a lot of antigens are actually sugar molecules) etc.

Just wanted to add that proteins incorporate many sugars for signaling purposes: either glycoproteins or proteoglycans. For example: Secretory vesicles in the Golgi cisterna use mostly mannose, galactose, and NAG (N-Acetlyglucosamine) to make the necessary glycoproteins for export.

Oh, and all those glycoproteins and glucosamines in the extracelluar matrix: albeit mucus!!!!