There is certainly a use for this, but I'm debating including it in most of my DD setups as my third atma on THF.
So, assuming you start with RR/VV, we've had a few discussions recently on the advantages of GH, ScorpQueen, etc., but 15% triple attack is also pretty nice.
That's almost a straight 30% increase in damage (i.e. 15% triple attack is 2 extra swings each time--which also get TP to WS faster), downside being if you get too many triple attacks you'll be wasting TP more often with 125 TP WSes.
Gnarled horn is an increase of 20% to crit hit rate which increases both TP and WS (assuming evisc). To calculate TP would be easy (if I had a parser)--take a typical party and compare the average damage of each TP swing and compare it to the average damage of a crit hit. If it's double, it's a 20% increase in TP damage. WS damage should mirror that. Also, wiki says the counter rate is 10 to 20%, but it seems far lower than that to me. Any info on counter rate?
So GH would be very close (depending on ratio of damage a crit/non-crit and couter rate).
I'm also assuming here that TA takes precedent over DA (I can't ever keep track of where the status of that is).
I'm also excluding the 50AGI from GH on trick attack damage since, well, rarely do I seem to get a chance to trick attack onto anyone. Mostly because there are so few melee DD on a mob at once/sides being important/etc.
Unlimited RR is very nice, but there are definitely situations where I wouldn't want to bother with it if I can convince myself that GH can be better DD wise.