I'm with Steeltrap here, the main reason for my love affair with SH3/GWX is down to the atmospheric gameplay which helps immersion hugely. The walking through the sub and being able to interact with some other characters, I feel, would be a novelty thing at first, but due to the constraints of being a computer sim, would become a repetitive exercise.
I would much rather the money,time and expertise to be spent on more imaginative and realistic AI behaviour. I don't want to feel I'm watching the same film over and over again with the same characterisations. The ability to have to come up with different plans and approaches to a given situation, and seeing the differing results, good or bad, that arise from your actions is what does it for me.
Just my tuppence worth.