Onyx (i) is useful at solo Armadyl, as the Defence speeds up kills faster than an extra +2 Ranged attack.

 

There's the answer to your question, it's better almost anywhere that you're maging or ranging and taking enough damage that Defence is needed. If you're meleeing, berserker (i) or vigour will almost always outstrip it as there aren't any monsters in RS where Attack bonus is more useful than Strength bonus when meleeing.

 

For example, if you were to DK without a cannon, not killing Supreme, it'd be better to use an onyx (i) for Prime/Rex. Once you kill Supreme though, it'd be better to use berserker (i)/vigour, and if you were to cannon prime, you'd be better off with a seers (i).

 

That's all well and good.

 

At Nex however, the amount of damage THE WHOLE TEAM takes depends largely on how fast you can take down her blood/zaros phases, and quite a bit less on individual bonuses. And i've been in teams plenty of times where we've had to do an extra blood siphon because we fell JUST SHORT of being able to get her to call Cruor, in which case the +2 ranged attack to the whole team could mean the difference.

 

Trying to compare Nex to any other boss in RS right now just isn't possible. She's a whole new ballgame, as far as bosses go.

 

I would agree with you though, that at solo armadyl, an onyx (i) would probably be the better option, as you are the only one taking damage and Kree has far lower defence than Nex does, so the range attack bonus matters less.

Ring of vigour is also a possibility as a switch (and in that case you *would* want archers (i) as your other ring) for clawing. But keep in mind that the ring of vigour only saves you 35-40% special bar bonus per kill, not enough for one extra claw spec on its own. It does save its own inventory slot in the form of recover special doses, and +3 strength is somewhat comparable to +6 slash, but it is unlikely to affect the quantity or timing of your claw specs within a kill. A ring of vigour's usefulness rises as the team size shrinks and kills become longer.

 

Also, defence bonus is not completely trivial. Nex *does* miss with mage a noticeable number of times per kill and misses with melee quite often (although it's very rare for her to miss with range, meaning you'd save maybe one low hit of damage with archers i). More misses means less food used as well. It's not a flat out choice between +2 range bonus and nothing. Even if +2 range is more useful, the difference in food saved during the ranging portions of Nex kills is smaller than that +2 number would usually save.