This reminds me of a conversation I had with a sound engineer earlier this year. I wanted to use a passive DI box of my own design, but similar to the one we're discussing here, to mono up a DJ mixer to feed our PA. He insisted that that a simple passive network wasn't the way to do it, and that we'd end up with cancellation and phase problems (I can't quite remember how he phrased it). If it was anyone else I'd have dismissed this suggestion out of hand, but the engineer in question was an old hand and highly experienced, so I have to at least give his suggestion some serious consideration.