Matthew,

Properly designed three chip projectors should not have convergence problems (though it's certainly a manufacturing issue). The advantages to three chip designs are:

1. Potential for better color fidelity and greater color control.
2. Elimination of artifacts that can be introduced by spinning color wheels. Faster and differently designed wheels have reduced the "rainbow" problem and even eliminated it completely for most people, but it's still an open issue for enthusiasts and a highly sensitive minority.
3. Potential for slightly quieter operation (the color wheel makes a high pitched noise in some single chip designs).
4. Potential for higher light output (not necessarily needed for the home market, but often desired anyway).

-avi