[Veselin Kolev]

Well... driver dificulty is limited to the gear ratios you use. For example, you can have 1st be 3 ft/s, 2nd at 8 ft/s and 3rd at 13 ft/s. You can always have crazy top speeds too, like 25 ft/s, but as you said, nobody can control such speeds in such a small playing field. And you can't really think of a robot as a car because you are using electrical motors, which work on a very different power curve than a car engine. A three speed transmission would just be like any other team's 2 speed, but with a mid gear, not an even higher gear.
One good use for 3 speeds is when the weight of your robot changes. Like the 2002 game, where you could grab goals and become 360 lbs heavier (if 2 goals are grabbed). Here, you could have your 2nd and 3rd gears act as your normal gears when not attached to goals, and use 1st and 2nd when with goals. This is why team 322 used a 3 speed that year. They grabbed goals, accelerated on 1st to 2nd and were fast with the goals.

Anyway, as for the complexity issue, I don't think thats the problem, but the problems that come with complexity, like the increase in the possability for something to die on you. As they say, the more complex, the more things that are to go wrong. But, this 'disaster' issue can be solved if you take a good look at your design and make it really rigid, and see where stress is the greatest, and fix that problem. The best way is to make a prototype and fix its problems in the final gearbox. But... how many teams have the time and money to make a prototype of a 3 speed transmission. Few. Not my team.