I like Vulcans! I love T'Pol. I think Spock suffers too much from being only half Vulcan -- all that angst gets boring -- I think B'Elanna suffers the same way for being only half Klingon.
After Spock did the Kolinahr and became more logical he was better, and losing his memories in ST:IV was a hoot "Double Dumb-Ass to you!"
He was not used very well in "Unification" -- I have to agree. The meeting of Spock and Data should have been much better. There was a problem with time restraints -- he was also involved in ST:VI at the time -- at least that is the excuse given. (It could never be poor writing!)
As to if he is in ST:X? -- I doubt it, too busy, would want too much money.
Will there be a convincing reason for him not appearing? -- John Logan claims to be a big ST fan. He wants to put in lots of little references for fans. If he misses that glaring plot hole then he has failed miserably!