I'm wondering about the same thing now. I'm also wondering whether the companions that aren't noble are worth making lords. Maybe just 1-3 of them, so that I keep everyone else in my party?
I'll try to categorize the skills.
Strength skills: Power Strike, Ironflesh
Agility skills: Riding, Shield, Weapon Master, Athletics
Intelligence skills: Path-finding, Tactics, Trainer, Surgery(?), Wound Treatment(?)
Charisma skills: Leadership
Useless skills: Trade, Prisoner Management, Persuasion (?), Inventory Management, Looting, Tracking (?), Power Throw (?), Power Draw (?), Horse Archery (?), Spotting, First Aid, Engineer(?)
Strength and Agility skills seem useless to me. The reason is that they help for powering up a single troop - the lord. I'm pretty sure that +8 troops (+3 charisma and +1 leadership) are better than 10 levels spent powering up the lord. However, if you have excess free skills, they might still be invested in some of them. Also, the Lord is more useful in battles where the number of units participating is limited, but still not worth it IMO.
I want to know whether there might be any use of the skills with (?) after them. I think that companions should be leveled in Charisma and Intelligence only. This way, from the Intelligence skill point bonus, they should be able to level up a few combat skills too.
I also wonder if companions keep using the equipment you gave them before converting them into lords. If they do, it would answer the question of whether some of the (combat) skills are useful for them or not.