In a 2 party system, yes it should
No it shouldn't. Bob Taft is one man; not the whole GOP. He's not even that representative of the GOP anyway. Simply because he is a terrible governor does not mean the whole party is terrible, or that the Democrats are any better.
Let's look at this another way. Kathleen Blanco of Louisiana is currently the lowest approved Democratic governor. Do you think the Lousiana Republican Party as a whole suddenly looks a lot more attractive?
While your first paragraph makes sense, its not the way our strict 2 party system works. If one incumbent becomes very unpopular, then at the next election his supporters usually don't vote for the other side, but stay home, and his opponent wins by default since all that matters is who has the most votes, even if very low like Clinton in '92 with 43%.
Obviously I don't like the Louisiana GOP, but even if Blanco's base hadn't been evacuated, I'd still say that her opponent has a much better shot at winning simply by not being Blanco.