blocks speaker sound
I don't own the version without the backlit keyboard, but I highly doubt the difference is worth removing the back lighting. That is, I don't believe people would be complaining if the original version also had a backlit keyboard. Maybe the ventilation of speakers could be made better, but not remove the backlight.
eats battery
I haven't tried testing a battery cycle with the backlight and with out it, so I am unsure as to how much power consumption it has. But it can be turned to lower outputs and even off, to which I would hope brings its consumption down to nothing.
increases weight
A few tiny LED's and the thin circuitry along with it? Has to be a small amount of weight! I would like to know the difference on that one. 🙂