It may seem like that to you, but have you ever had a MCH in your party that uses both Reassembles on their AoE skills and/or an early Hypercharge? Or a SMN using Bahamut with Daethflare and Ahk Morn? Maybe they wouldn't die immediately, however it IS very easy to rip off aggro from a tank during the pull, and more likely the further away the "wall" is. (and yes, using said actions so early before you reach the wall may not be optimal, but some people/dps do it anyway)
Add your suggestion into this, it will either result in smaller or way slower pulls because the tank has to establish and hold aggro with single target skills. And as I said before, if you do not intend to widen AoE capabilities/complexity/etc. for other roles, trash pulls will remain braindead for them while increasing the amount/time of trash pulls... this is not desirable.
Maybe I don't understand your intentions here. Can you explain it in more detail?
As long as it's not a times 10-multiplier, but less, anything will be better. Before we open up a discussion with duties (Eureka, Southern Front, dungeons/trials/raids), buffs (brink of death e.g.), excel etc. let me just give an example.
Let's say, there is a DNC choosing a dance partner before the pull. They want to determine whether their dance partner decision was right or not (assuming you don't have access nor using to certain 3rd party tool);
currently, they can only "confirm" their choice by looking at the enmity order/number shown, but the enmity bar is basically invisible.
imo, they should see the enmity bar and determine whether the difference - compared to the next best dps - was worth choosing their partner or they should swap at the next best opportunity.
...it's... not the best example, but the point is:
The enmity bar should not just be visible for tanks with stance on/shirk used only, but for dps and heal, too... like ~max half or a quarter of tank aggro... something like that.
Otherwise, why even bother keeping it? Just get rid of the enmity bar! Essentially, it serves no purpose.
PS: Dead Ends is a bad example because you get item level synced, and apparently Sqex is reworking story content with iLvl sync, too. I still get your point, tho.
Edit: Nevertheless, it should NOT be possible as iLvl 42-45 NQ GLD(!) with Iron Will to hold aggro vs an iLvl 130 synced SAM in a lvl50 boss fight... the dps has much better gear than the tank, even with the tank mastery trait and mitigation in mind they should take less dmg than the tank. (true story btw)