When combat occurs, it also defends saurons plots. Because of combat, hero have to kill that weak monster or be stopped in place. When sauron is lucky to get a 3 strength 5 attack melee card it is also very good to deal 3-5 damage to hero. Only by playing shadow card. You think combat with weak monsters is not worth it ? Your problem.
But this is only if Plot + monster are in the same location. If you send 1 Hero out hunting for monster tokens, seems Sauron in your games will spend an action to move the monster, so it is not forced to Ambush and get discarded.
As to Reckless, if I'm against the fodder-Crebain/Agent, why not go with Parry first? It'll kill either of the two, because of Reckless and you take 2 dmg yourself.
Mission card should not be that hard to fulfil.
It's not like every Hero Mission is hard. They have 3 very easy ones (Spear of the West, Noble Blood and Isildur's Secret), 2 hard ones (Against the Shadown and Keep Minas Morgul at Bay).
For Sauron, there are 2 Missions the Heroes can nerf on their last turn (or even on the Hero Rally Step on the turn Finale starts) that he can do nothing about (His Dark Throne and To Find Them All). Even his other 3 are solely dependant on the Plots he draws. Maybe he has the Black token to stage III Mission, but no Black Plots come into play until very late (which there is a good chance of, as the good ones require 5 and 8 influence in the Shadow Pool), but he did draw a lot of Red Plots early and could start the Finale without completing his Mission, but with dominance. I'll take final combat with dominance over trying to complete my Mission if that can result in the Heroes tying for dominance. Like you said, Mission is secondary, dominance is the key, if Mission is completed, nice, if not, if you took care of dominance, doesn't matter.