So your argument is that he only fights enemies that he knows he's going to lose to? How does that make any sense?
And if he's truly omniscient, how does he get surprised by anything? Omniscience implies that he knows how everything is going to turn out, but he doesn't. That's why he gets angry, or afraid, that's how he gets beaten or trapped by cosmic cubes or 'killed' by Dormammu, defeated by the Gauntlet, etc.
These beings aren't omniscient, as portrayed. They're little more than CHILDREN, as portrayed.
It's indeed possible that he could be Omniscient and still experience emotions in response to stimuli, he'd just know that he was going to feel those emotions beforehand and be unable to change how he feels, assuming Marvel is determinist that is.
Dr Manhattan from Watchmen for instance (not that he's Omniscient) knew that Laurie was going to tell him she slept with Dan but when she did tell him ten seconds after he told Laurie she was going to tell him he still felt hurt by the "revelation".
Not that I really think Eternity is Omniscient in the truest sense of the word, I think he might be Omniscient within the confines of 616.
__________________
“Where the longleaf pines are whispering
to him who loved them so.
Where the faint murmurs now dwindling
echo o’er tide and shore."
-A Grave Epitaph in Santa Rosa County, Florida; I wish I could remember the man's name.
But that's exactly right -- Dr. Manhattan ISN'T omniscient, and the entire crux of getting one over on him (ie, surprising him) was manipulating tachyons so that he DIDN'T see what was happening.
Eternity's anger came out of NOT knowing how things would play out, about desiring something or being dismissive of some small being (a small being who would eventually be important to the overall outcome of the conflict).
While I don't think showing emotion is indicative of a lack of knowledge, I do think being dismissive of a being who is ultimately going to have a heavy affect on the outcome of your big war indicates a lack of knowledge of the part that being is going to play, which would preclude omniscience. Same deal with being taken by surprise by the Cosmic Cubes of Magus, for instance.