ACtive D-Lighting applies and exposure adjustment as follows: Low 0.0 Medium -0.3 High -0.7 Extra High -1.0
The exposure adjustment is baked in and cannot be removed in Post.
The tone curve balances the exposure to correctly present the image for JPEG files or if you are using Nikon software (or other programs that use camera settings).
The tone curve can be adjusted in post in Capture NX2. If ADL is turned off, an adjustment is not possible.
The ADL curve is relatively complex - I am very experienced with editing and found it was not easily replicated in post processing with 15 minutes of effort. In simple clean scenes you could probably replicate the effect in post, but when you have a multi-toned background or busy composition it can be very difficult.
There is a very slight slowdown in writing NEF's to the memory card with ADL turned on. I could not observe a difference.
Since ADL does affect the embedded JPEG, it will change the histogram in your LCD. Your picture control settings also impact the LCD image and histogram. This can mean more or less clipping depending on your settings, and lead you to different exposure adjustments.
If you shoot NEF's with ADL on Low, there is minimal downside and you retain the ability to adjust ADL later. It's a factor in high contrast situations with complex scenes or backgrounds as you might need the flexibility and you get a histogram that is closer to what you might render in post.