Lets take the current Connecticut democratic primary.
It’s a safe dem seat, so while I personally favor Lieberman to win, I believe a primary is warranted.
The application of this particular "rule" would be problematic. For example, if you support the incumbent, you may claim that a seat is only "safe" if the incumbent is running, while if the "crazies" in the party get a victory, the seat may be lost.
And oddly, none of the usual suspects in the comments has mentioned last year, when Steve Chapman ran a primary against Parrish in what has to be considered one of the safest of republican seats. Some who are now saying primaries are good things didn't like Steve's running a primary campaign.
It was last year, during that campaign, that I wrote about why primaries were good for our democracy. And while I disagree that they should be restricted to when a party's hold is sure, I did say that primaries were most important in that instance, to give voters SOME method of signalling their opinion. From that column, titled Primaries offer competition instead of business as usual:
The Democrat and Republican parties are having primary elections on June 14. I am a strong supporter of party primaries. The way most districts are drawn, a primary is the only chance to make a real choice. Incumbents are not entitled to their offices, and democracy is best served by healthy competition. We hurt democracy when parties suppress primary challenges, and when the media paints them as a negative. Voter choice is a good thing.
Of course, primary challenges as a practical matter cause trouble for incumbents, use money that would otherwise be used for a general election, and can leave bitter feelings between groups. But that is more because of HOW we run our elections, rather than the elections themselves.