In reply to minimike:
In my experience:
- My Mantas are made of thick leather with few seams, no Goretex and are totally waterproof, whatever Scarpa say.
- My Meindl Lhostes are made of thinner leather with more seams and a Goretex lining. The lining failed after about 2 years, but regular leather treatment made them nearly-waterproof again (until the stitching started to fail after another two). I bought them 9 years ago, so technology might have improved.
In my opinion there is no point putting goretex in a boot like the manta, it's already waterproof. Lighter boots are less waterproof, so might benefit from a liner, but the liner will probably fail before the rest of the boot. For lightweight boots, made of canvas, Goretex or similar is probably a good way of being waterproof.
Adding a membrane will always reduce breathability compared to not having one, but I'm not sure the leather of anything but the lightest boots breathes enough for this to be an issue.
I think marketing has a lot to do with it too. I'd personally pick leather over gore+leather, but might consider gore+canvas for summer walking.