Ya concepts like skill grind and grinding in general were just methods to stretch out the fluff of content. It's hard to develop an MMO under the idea of developing skills because even though you might seem to have options the reality is unless you follow certain formulas you are hurting yourself cause the best setups are usually found quite quickly and deviating from that set path alienates you from the community.

In a single player rpg, the concept of developing skills is easier because you have control over a unit, it's not just you playing by yourself and relying on the skills of others but rather the ability to develop your team in your own way and easily being able to adapt based on how you grow their skills.

It's hard to do that in an MMO concept because the way you set yourself up may not be compatible with the other people in the party and you just become a hindrance to the group.

With that in mind though, the idea they mentioned that I imagine they are using to keep old dungeons relevant wherein you control a unit and bring them through the dungeon, those characters could be given an in depth skill tree/customization because you can develop that group of 3-4 how you like and adapt their skillsets based on the overall group.

With unit based that you control it's easier to add customization because the variable of player confliction is nullified and it just turns into a single player rpg experience which in all honesty it will be fun to see how they implement these soldiers/mercenaries and how much control the player will have over them.