I want to say this has been answered already, but one reason your DSP (as well as most DSPs) has 6 inputs is occasionally a signal is split up into highs, mids, and low and it requires more than 2 channels to construct a full signal. This is generally an issue for stock systems in which a crossover slope is applied near/at the front end. One could also argue so you retain fader and balance controls, but really if you're in the DSP crowd you likely won't ever touch those since it will throw off your tune.
...As an aside someone may run into the extremely esoteric scenario where 1 DSP isn't cutting it and he or she thinks they need 2 DSPs where the multiple inputs can come in handy...SQ people can be crazy sometimes.
In short a DSP generally has 6 channels to remain flexible. One would think someone with your illustrious background wouldn't need so much holding of the hand. ;-)