Are talents like 'Weapon Mastery: Knight' worth it for fighters? Or are the talents better spent else where?

I suppose that it may depend on the role the fighter in question is playing.  If his role is to be a defensive fighter, a tank, then the talent points may be better spent on defensive talents, though if the fighter already seems well skilled defensively, weapon mastery isn't a waste, I'd think.

 

OTOH, if the fighter in question is a more dedicated offensive focused fighter, then I'd say that weapon mastery is a definite yes.  An additional 15% (IIRC) damage all the time seems like a no-brainer to me.

Crucis has already covered most of it. I'll just add that I found a good tank pretty mandatory in the early game and so I started off Eder's build as a tank (assuming your PC isn't built as one and you're not hiring one from the inn). Later on in the game, most of the abilities and talents that improve Deflection will already be taken, so you can either go with abilities/talents that mitigate damage even further like Critical Defense or go with talents that improve damage to make him a bit more offensive.

 

Pallegina can serve as a good tank as well and in some fights with small chokepoints and enough crowd control, you really don't need more than Pallegina plus an animal companion as offtank. In those situations I was happy to let Eder dual wield sabres in light armor/clothes and weapon mastery would certainly help there.