It's always been my impression that ini is considered pretty junky as a defensive stat and not worth focusing on, but since I had the Dom and Rare Fortune set bonuses on my Runepriest, and several ini talismans rotting in the bank, I decided to run a little experiment and stack it myself and see.
All combined, talismans, ini from gear and set bonuses, renown points on the ini stat, and full points in mid-tree to cap the RP ini buff (+80), gives me a grand total of 779 initiative. I suspect I could get it somewhat higher but probably not by much--at least, not on a Runepriest.
With no bonuses from gear and no points in futile strikes, that gives me a 4.5% chance to be crit (if the paperdoll is correct).

Not so hot, right? But ini doesn't just reduce crit, so how about bonus to dodge? Well, it does seem to provide a modest amount of that (have ~29% dodge with no gear bonuses and no points in dodge-disrupt), but it's my understanding that the strike-through provided by ballistic skill works against the dodge provided by initiative, and any ranged class worth their salt has far more than 779 Bal. That said, I don't know if the strike-through works on a 1-1 basis, that is, whether 100 bal cancels out the dodge provided by 100 ini. Can probably dodge the heck out of thrown daggers though

Beyond that, initiative also provides a chance to detect stealthed enemies. Since I do feel like I detect Witch Elves more often and from further away at ~330 ini (my normally geared RP) than I do with ~200-250 ini on other characters, I had some hope here that initiative stacking might provide some laughs by giving me a reverse x-ray vision to see into the secret world of Witch Elves.
Unfortunately, in some (admittedly limited) testing I don't seem to detect WE's that much more often at 779 Ini than I did at 330. So if Ini provides a bonus, it's low enough that it's difficult to notice a significant difference by stacking it.
Some questions I'm left with:
Does crit reduction provided by ini scale logarithmically(?) at very high levels (approaching soft-cap) as it does as very low levels?
How does strike-through provided by main damage stats scale against defensive stats?
How does stealth detection work generally, and how much does %chance scale with initiative?