Why not have sets include both an offhand and shield? Then if you want to go with a shield build you have that option and if you want to go with an offhand you have that option.
The two items would have different stats, one geared towards tank things and the other towards casting things.
For example, I wanted to use valguur’s set for a tank build, but it has an offhand and not a shield. Though it’s possible to tank without a shield, it’s a different playstyle from having a shield, as well as different devotions for shields vs no shields.
dreadwalker is another i wanted to use for shield tank, but it has an offhand.
it’s nice to have options, why not have both?