I can see the reasons for this argument, and it's not without merit. However, though I've heard the argument many times, I've yet to hear of anyone that has actually followed through with it.
For example, let's look at the current Republican presidential primaries. There are no Democratic presidential primaries this year. Since there is a chance that on of the winners of the primary could become president of the United States, and since the stakes are so high (as we're told they are every year...), it would only make sense for Democrats to register as Republicans and vote in the primary for the best choice. Sure, none are ideal, but lesser of two evils and all that. It won't stop anyone from voting for Obama in the national election, and if he looses, at least the best GOP candidate will be the one replacing him.
What do you lose? The ability to vote in the Democratic primaries? Well, keep in mind how dangerous we're told some of these candidates are. If we follow this logically, it only makes sense to register as Republicans. Besides, if you live in a Red State, then voting in the Democratic primaries would just be a throwaway, "protest vote."
Is this the right thing to do? I honestly don't know, but I do know that it makes sense from the "lesser of two evils" point of view. Surprisingly, people that advocate the "lesser of two evils" approach don't do this themselves.
There is a possibility, though, that such an approach is more about intra-party discipline than about actually making a change.