I'm not sure weight is the big issue. I like to keep everything simple though, both physically on my handlebars and with the cables. I also never saw much of a use for them, if you're on your tops you're probably just cruising along and shouldn't need the brakes.
I had them before and never really used them. I did toy with the idea of trying to somehow putting a top lever on the left attached to the rear brake. maybe for some control for tricky dismounts but never tried it.
I kept the front one installed so I could brake while riding on the tops one handed heading to the pits with my pit wheels. otherwise, I never would use them and only had problems with them behaving like a mud/grit portal into my already drag-filled brake cable housing. If you ride a lot on the tops you might want them but if your bike's set-up to handle comfortably from the hoods, you might feel like your hands are too close to you and to each other when riding on the tops to get any serious work done. That's how I feel anyway. When I change my front brake/cable/housing I wont be putting the Top levers on.
I always thought they were cool until I noticed that the top riders don;t use them, so I don't run them on my bike. Just seems like another thing to have to clean/fuss over. Clean handlebars are the way to go. If you're worried about handling on tricky descents consider going with a wider bar, that'll give you a little more width on the tops which might give you more control.
another often-forgotten benefit is that most levers give you a nice option to adjust cable tension - which is great if you don't have that built in to your brakes (increasingly common) or hangers. it's a nice quick adjustment based on muddy/dry conditions.
Personally I think that if you are not in your drops or occassionally on the hoods in a CX race then you are not racing, so the levers I think are a complete waste of time. I feel that I have way less control of my cx bike when riding on the tops of the bar so the last thing I want to do is brake from that location.
Here's how I look at it...If you like them & you will use them, put them on, If not, then don't. I came from a MTB background so I liked being on the tops of the bars a lot. The more I got in to CX, the less I used them. I have thought about putting 1 on the rear brake for fun but have yet to do so. There have been times when having top brakes would have helped though....