Eh, I think Trusts is the better between the two.

We can argue that it's 'double the work' all we want, but ultimately the people scripting these may not necessarily be the same people spearheading other parts of content, and frankly with a solo option you do limit what you can do design-wise, and it makes transitioning from 'solo' to grouped content more difficult. - Let me put it this way, with solo you don't have to care about anything else but your own position, at least with trusts you come to appreciate this more. Equally, you end up developing bad habits because either you won't have stack/share mechanics, or they will be something you take alone which only aids in developing bad habits, e.g., running away with the stack marker rather than taking it in to people. - You then also put people in a position where they can just eat mechanics for breakfast because they will be taking on that multi-role. They'll have the damage of a DPS, the vitality, magic and physical defense of a tank, and a basic healing toolkit to boot. It's a recipe for developing bad habits, especially when you look at implementing it for more general content, e.g., MSQ.

I think the advantage of the presence of trusts far outweighs any development costs.