Ti'Tahk also has an appaling proc chance (50-80 seconds between 2 procs) and 18% uptime. I somehow doubt it's ever worth weapon swapping over DTR. In my current gear, as demo, I sim 350 dps higher with Sho'ravon than Ti'Tahk which makes both staves pretty even. Hopefully the heroic version has higher proc chance.
I have noticed the crappy proc rate as well which leads me to believe it may be on spell-cast opposed to on spell damage or healing. I didn't really play around with it too much to figure that out. But it makes sense that even with a 15% proc rate (which most items have) it may take 10-15 seconds to get it to proc if it holds to that 15% and in most instances will take significantly longer. In the middle of a fight, I'm generally putting out 5-6 actions per 10 seconds where, with most other trinkets/procs that will trigger off of damage, will be putting out double to triple that.
I also have yet to see a random proc from soul harvest, health stones, etc...
Really I should just run a combat log and figure out when and off what it procs but I have limited time for my warlock during the week.
EDIT:Apparently the proc rate was hotfixed as it is proc'ing all the time now (usually the first thing to proc) and off of all the aforementioned things that I said it has yet tod do (soul harvest, etc...)