Mice are useful in studying taste because strains have been bred with the inborn ability to taste or not taste certain bitter substances. Studies of these mice have pinpointed a cluster of gene positions on mouse chromosome six that are associated with the tasting of a number of bitter substances. Everything so far hinted that T2Rs were bitter taste receptors, but the researchers still did not have definitive proof. The olfactory system is designed to recognize a wide range of odors and to discriminate one odor from another - an essential ability if one is to avoid such inappropriate responses as mistaking a mate for a snack. The organization of the olfactory system reflects this need, with each olfactory neuron expressing only one of the 1,000 or so olfactory receptor genes.
Taste is a different matter, especially where bitter compounds are concerned.
The important thing is to recognize and reject anything bitter, not to get hung up on distinctions among different compounds. Indeed, experimental evidence indicates that humans are unable to discriminate one bitter substance from another. Every cell that expresses genes in the T2R family expresses nearly all the genes in that family. So rather than having one receptor per cell, like olfaction, you have many. This dramatically increases the repertoire of bitter things you can taste, but since the receptors are all in the same cell and the cell simply fires when activated, you do not discriminate.