Personally I have ridden too long to add that gadgetry to my bike. People say that levers up top are good for braking when coming to a barrier but I am comfortable using the rear brake with a regular lever set up. Besides the fact that I do a majority of a race in the drops. And lastly, why have extra weight? Speaking of levers, if you have a hard time using the right lever prior to a dismount you can change the right lever for front brake and left for rear.
My new bike has them as did the bike I used last year and I found I only used them once, on a pretty steep and muddy downhill section. I think I'm probably going to get rid of them when I redo my handlebar tape. I don't think the extra weight is worth it, having only used them once. The idea is good but they put my hands in such a narrow position that it ruins the stability factor. I've found I'm usually more confident on steep sections on the hoods.
I had top mounted levers for a little bit, but I ended up never using them so they're gone. I found that it throws my position completely off kilter. I would say ditch them and just learn to ride more comfortably on the bends.
It probably also depends on the courses you ride. On the East Coast we don't have too many crazy descents, but in places that do they could be a lifesaver I imagine.
Dieter Runkle (sp?) was first to win Worlds with clipless pedals (clip-in) and top mounted brake levers...for the mountain bikers it seemed like a good idea. Custom drilling a set of ritchey or dia comp SS5 brake levers may have added some trick bling to your cross bike "back in the day".
Different riding styles will have different beliefs (I hate the whole "hell no" concept because some people have individual needs not limited to decreased hand movement etc.). Some people swear by them, and those people are right---for themselves.
I do find them a crutch: When descending with the extra levers, I would find myself on the tops (not the best place to be IMO) and using WAY too much brake. After I got rid of them I kept my hands on the hoods (or drops) and never looked back. Also, many people say they like them to modulate speed coming up to a barrier...to me, that equaled slowing down too much before the barrier.
They aren't necessary, but a lot of rooks and old fogies like them when their backs get sore halfway through the race.
I would say use them if your technical skills are not up to par and drop them when you can ride / race comfortably without using them.
I use them. I find them useful only when I'm trail riding with my MTB buddies and the trail gets too technical and downhill. I don't like riding in steep, muddy downhill slopes with my hands on the hoods or the drops. Makes me uncomfortable as I feel like i'm too forward in the bike during the descent.
I never ride on the tops in cross or trail riding. In fact when things get sketchy my instinct is the opposite, to go to the drops. Maybe it's the roadie in me. I figure those things must be a mountain biker thing to mimick a flat bar position.
When I re-cabled my rear brake I ditched the rear lever. I think it was allowing more grit and moisture into the cable and part of the mechanism was binding and not releasing the brake completely. I didn't need to re-cable the front so I left that one on and I'll probably leave it on since I ride my spare wheels to the pit and like to keep my left hand on the top of the bar so it's closer to the center of the bike and more stable. Also it looks strange and I like that.
I never leave the hoods when I race (occasionally I hit the drops) so I can't comment on their usefulness during the race, but they do seem to complicate things and shifting hand positions in order to brake seems like a questionable call...