This tactic has become so common that the Google reps will actually explain the concept to advertisers now. When the reps are willing togo that far and actually endorse using this tactic on occasion - seems thats a cue for Google to change how the system works for content match bidding.
The Google system also likes to experiment when you have the same keywords in multiple places in your account just to see if the other set gets a higher CTR.
If your content campaign starts to get a better CTR than your search campaign, then it can start to show on Google instead of your actual search campaign.
By keeping the bids far enough apart, you can minimize this crossover effect as in essence, you're manipulating the ad placement formula ( [webmasterworld.com...] post 11 for more info)