Here is what I have learned from my own parses (using Guildstats.net latest stable build) as well as the honorable Slith and Cynara. I play Conquerer and Sin primarily.
Offhand procs do vary; if my percentage is 43% offhand chance on my Skills tab, then I can see it happen as little as 30% of the time or as often as 60%.
There is a base fluctuation which seems in the 10-11% range that just makes sense. Factoring in the enemy's evade rating makes a huge impact. Further, Guildstats (and as far as I know, ACT) both fail to register offhand hits resulting from procs/combo finishers, only listing white offhand hits under the "Offhand Damage" category. Corrections welcome with citation.
I've done several 100 white hit tests on straw men (base 3% evade, same base rating as the average mob) and seen a huge difference versus, say, even a level 80 Khitai mob; a level 80 Khesh mob is very close to the 3% evade figure above. Khitai mobs seem to sit in the 6-9% evade range with numerous exceptions. Remember, bosses are worse, and many mobs can decrease your hit rating as well as buff their evade rating, meaning fewer hits = fewer chances for offhand.
I've also tested offhand procs on all DW combos including the base ones - yes, this means going through the log line by line - and found that each combo has a very specific internal likelihood for offhands both during white loading hits and the combo finisher. Slam almost always gets offhand for me (Throat Slash gets 3 hits with DW, 2 with 2H), Whirlwind has a high offhand, too - they're in the 50% range. So what combos you do will determine your overall offhand output.
Remember, offhand hits *never* crit, and will never do as much damage as a main hand hit.
Conclusion? My offhand rating is in the mid 40s and I land, as best I can tell, about 40% of my swings (slightly less) get an offhand! I can't find anything inherently broken about the offhand system in the game, and a 5% uptick in offhand leads to a near equivalent if not greater increase in DPS (more hits = more pain!), so stack offhand, kids! It's practically FREE DAMAGE and can really make the difference on high evade enemies!