What are the advantages of a long knife?
A very long blade is only useful in specialty situations, such as cutting very thin slices from wide cuts like an entire top round, or a full sized ham where there is no bone to be negotiated around (in the ham, the slice is only down through half, until the bone is reached).

Otherwise, for more typical home carving purposes, extra length just gets in the way, especially for oddly shaped meats like an actual turkey.  I find that a traditional chef's knife or my santuko are the best tools for most home carving jobs&mdash;and they are also useful for many other purposes.

I have a 10 inch (25 cm) granton ham slicer, but I never use it for carving:  it is reserved for slicing layer cakes in half horizontally in one pass.