Open your Inventory and click the Merc tab. Below, and on the right, you will see the 4 Mercenary inventory slots. Move the gear slots up, or down, in order to create space for resists to be displayed.
Why? Merc gear doesn't have resists on it. So the only way that resists can be modified is via buffs. And honestly, who's casting resist buffs these days?
Why? Information is useful. This is not a discussion about Merc gear. This is request to have more information disclosed regarding Mercs. Resists can be modified with AA's and Buffs. Having a visual representation of your accomplishments, in this case they take the form of Mercenary AA's, being represented just as yours as a character are. You no doubt know the max number and confirm you have accomplished that task by seeing the number in your inventory (or if you haven't, you are working to get there). This applies to Mercenaries also. It is important in that regard alone. Simple to some, good information to others.
If I go AFK in the lobby for any length of time, I always come back to a full set of resist buffs on my toons. Obviously, someone is still casting them.
Does merc resistance matter at all? I've never noticed an issue where I was saying, "Gee, I wish my merc had better resists". I also ignore resists on my character at this point as, except for Corruption, they are all maxed. What I want from my mercs is better AI, control over their (stupid) actions, and a tank merc that isn't a joke.