Marsu42, In checking my copy of Victoria Brampton's book for Lightroom, here is what she says. (Victoria is one of the most knowledgable Adobe users and frequently helps photographers out on the Adobe forums as well as publishing books about lightroom.)
"Canon’s Highlight Tone Priority automatically underexposes the raw data by one stop to ensure you retain the highlights, leaves a tag in the file noting that this setting was applied, and applies its own special processing to the JPEG preview that you see on the back of the camera. Lightroom understands that tag and increases the exposure by one stop behind the scenes to compensate, but if you accidentally underexpose the image with HTP turned on too, you can end up with a very noisy file. When shooting raw for use in Lightroom, there’s no advantage to using that setting instead of changing the exposure compensation yourself, so you may wish to turn off HTP and set your exposure to retain the highlights manually.Canon’s Auto Lighting Optimizer and Nikon’s Active D-Lighting don’t affect the raw data itself, but Lightroom has no idea that you’ve used those settings, and even if it did, the processing applied by the camera is variable. When ALO or ADL are turned on, that special processing is applied to the JPEG preview that you see on the back of the camera, as well as to the resulting histogram. Seeing that false brighter preview could cause you to unknowingly underexpose the file, and then be disappointed to find it’s very underexposed when you view the raw photo in Lightroom, so it’s a good idea to turn those settings off unless you’re shooting JPEG or only using the manufacturer’s own software."
So, its pretty much worthless or even detrimental to most users of raw, and should be turned off unless you need it for straight to jpeg shots. You can always under expose bright scenes as you see the need if you are using raw.[/size]