I think the end result here is that tanks are the only job that have to make the decision to nerf part of their effectiveness (the role they were actually brought along for, technically) in order to be optimal.
Healers and DPS continue to gain power in their DPS output AND their survivability via constant upgrades. They don't have to choose, every upgraded ilevel part they get just buffs them in their role.
Tanks have a choice. Choice is good, yes, but it's weird - on gearing ourselves to be "optimal" by putting out damage, we're making ourselves have a lower EHP. "Harder to heal" isnt really the term, but we're nerfing our ability to be left alone without heals, survive when healers are dead, or take even more spike damage than intended.
Of course once you're comfy in full slaying then yes, every item is an upgrade - mostly talking here about when you're still a bit under the comfy gear for an encounter and are wearing a mix of Fending and Slaying. You get an upgraded left side part, it boosts your damage and your EHP... and then you nullify the EHP boost by swapping out a fending for another slaying.
It can be fun to fiddle the tradeoff, but as you can see from this thread it's a contentious issue. Tanks want to do the highest threat/damage output they can. Healers/DPS want a faster run. But if the tank dies because they make a misplay or the healer has a hiccup, fingers will be pointed at the fact they had less HP than they could have had.
And all this contention just boils down to one thing - tank accessories. If Fending accessories didn't exist and all tanks HAD to use slaying, all would be good (just some cutting edge tankbusters would be a bit harsh). Likewise if Fending had strength on them, or tanks scaled with vitality, it would be fine too.
The issue at hand is that feeling of having to choose to nerf part of your power in order to improve another, which is a tradeoff that other classes don't have to make. Imagine if Cleric Stance didn't exist and healers had to juggle Mind gear for heals and Int gear for maxing dps. You'd see healers running dungeons in full int accessories and the healer forum would be equally awash with "omg healers please use Int accessories for faster runs" coupled with "stop using int accs in dungeons".
Having a choice and gearing correctly for an encounter is part of the fun/skill of an MMO, but it IS a little weird that tanks are the only class needing to make that tradeoff. I think everyone would end up happier if there was a one-size-fits-all mentality for accessories.