I don’t think it is as clear-cut as that.
In Gefjon’s case it is, I think for every fast sniper you probably want to enhance the killing chance of the sniper. However for a hero such as Mist, Proteus or Hansel whose “strength” is their high attack and who are damage dealers but who I would consider more of a tactical attacker I would say that unless you purely want them in a mono stack for their high attack value (like I used to do with Proteus) you would want to enhance their defense so that they survive long enough to be able to cast their special. Once I moved Proteus to a 3-2 setup he would die before using his special as often as not. I re-did his emblem path from attack to defense and now he can survive to cast maybe 80-90% of the time.
With heroes like Malosi or Zocc who are semi-snipers with a very important secondary effect you can go either way, depending on how you want to use them and how you are finding their survivability
With Lady Loki I have taken a similar approach, because whilst her up-front damage can be decent if loaded on debuffs, her main purpose is to cleanse and to debuff the enemy. The enemy is far more likely to die from reflected DOTs than due to greater up-front damage.
For healers or support you typically want to go defence or health, whichever makes more sense (typically I do defense first, for Noor I did health up so that her team minions are stronger). But if for example you find that you go mono and 3 tiles almost always almost kills the tank, then it may make sense to beef up your healer’s attack instead.
So broadly speaking I agree with you, but it is also dependent on your playstyle and objectives, the tyeps of teams you face and the challenges they present, and the specific hero in question, particularly when a lof of newer heroes don’t fall into a clearly-defined role