That was me.
I sent WARN an email asking why they do not recommend the line.
Here is the response:
<snip>
Warn does not recommend the nylon rope because our winches generate a lot of
heat during a pull. The nylon rope does not let the heat escape like wire
rope does through the drum. Without letting heat dissipate, the winch can
overheat very quickly and burn up and also possibly melt the nylon rope.
Sincerely,
Anne Snedecor
Senior Technical Customer Service Representative
Warn Industries, Inc.
<end snip>
To help with this possible problem I always try and use a snatch block or have tons of line out. This should help with the heat problem. I also just take my time and if the winch is getting to hot then a wait a bit. If you just run your winch then I could see this being a possible problem but it would also be with wire line (just might be faster with the rope).
The MasterPull line is (if it's not yet) going to be mandatory at Rock Crawling competitions (for safety).
For me the safety factor was worth the possible damage to the winch. It is also great to work with (you can use bare hands).
Just my 0.02