I can understand why they designed it that way - I would have done the same had I still been working in R&D at the time.
The reason I can see is that if you're holding, say, a C chord down and hit the Intro, the Left Voice will continue to play the notes that you're physically playing while the Intro (apart from some Intro 1s perhaps) will go through the chord sequence that the programmers came up with, something like C - Am7 - Dm7 - G7 at its simplest. All bar the Am7 would clash with the held C chord. A lot of the Intros are far more complex and would clash more wildly.
I think you're wanting to do something that the majority of players would not want it to do, and there's the age-old problem. You cannot design an instrument or put in a feature for a minority, however useful such a feature might. Been there in R&D, had a great idea, only to have it turned down as it was a minority request! Resources have to go into what the market demands. Great fun when coming up with new features, I can tell you, and the arguments in the meetings can be fierce.
Just my tuppence worth!