Worth is a very subjective question. My bet is you'll get both answers, yes and no. The price differences (which may well be substantially less than MSRP differences) will be more impactful to some people here than to others.
Best thing is go ride all three and see how much you can tell the difference between the models. Where you can discern differences in the various systems/features assess the value to you of those against the price difference in terms of its impact to you.
I ended up ordering an 8.6 a couple days ago and should get it early next week. Would the 8.4 have worked for me? Sure, it's a great bike. However I'd looked at all three models and decided the added convenience of the remote lockout was worth it to me, plus the higher level brakes and shifting just "felt" a bit smoother. Ordering during the TrekFest sale helped also helped as the credit got higher and lessened the price delta.
Last edited by gpburdell; 04-15-15 at 01:20 PM.