To put what Wart said in slightly different terms, the training for trimix dives does not have a whole lot to do with the trimix, and, yes, if you knew how to set up a program like V-Planner to give you the proper mixes, you would "know" all you needed. The training is first in dealing with the skills it takes to do those dives, such as switching tanks without changing your depth, switching to the correct tank, being able to make all switches and other skills smoothly and without having to look at your gear and thus losing focus.
Next, it is about dealing with everything that can possibly go wrong on such a dive. How much gas will you need in case of an emergency? What if you have a problem that makes one of your deco gases unavailable? what if your buddy has an emergency while you are in the process of doing one of these skills?
To give you an idea of what is done, fairly early in my training my buddy and I were, in theory, just going for a dive that was supposed to end with a series of deco stops (simulated--not really needed). We were starting that dive with doubles, an AL 80, and an AL 40 each. The instructor piled one failure on top of another, so that by the end of the dive, the only gas we had available was one AL 40, with which we were buddy breathing during the deco stops. My mask was gone, so I was blind. We were required to stay in good horizontal trim and hold our stops at the correct depth. As you can see, that experience has nothing to do with helium, but it has everything to do with getting to the surface safely after you have done a dive deep enough and long enough to require helium.
