timo... have you ever actually raced? i'm not trying to be a pain here, but if you've ever been knee-deep in freezing mud on the sixth lap of a december race, you'd understand that any moving part on a 'cross bike is added complexity. any moving part can get jammed. when i see top-mount levers, my first throught is "gee, look at that -- another place for mud to get in and clog your cables."
i'm probably not being fair to you, and i apologize. as far as i know, conditions in your area aren't as muddy as they are up here. my dislike for top-mount levers is probably due in part to the fact that i know that cables get iced up and jammed with slush and frozen mud.
the other thing is that, in my experience, the only time i'm ever in the tops in a race situation is in a long climb, and i've never needed to brake while climbing. the rest of the time, i'm in the drops or occasionally in the hoods. as far as i can see, top-mount brake levers on a race rig are about as useful as saddle-mounted gear shifting.