Well, I do believe that was his original goal. However, Xehanort started praising the same speech about darkness when he entered Terra's body. I'm not saying he didn't care about the balance, but Eraqus refusing to except the balance probably made Xehanort go insane and think that the current light was corrupt, only for Xehanort to become corrupt and totaly biased towards darkness.
He maybe did, but remember in front of him is a bunch of Light lovers peoples. It is his role to offer the 'good' viewpoint on the darkness, no one will do it if it is not for him. The others refuse to use Darkness, or do it reluclantly, so he'll forcefully speak of the advantage coming by using them. Someone has to fill the role of the Darkness user, to show that the balance *is* possible.
You have a point, but necessary does not equal good.
I'll have to agree, in fact I prefer to consider characters neither good nor evil (I don't believe in those words really in the first place).
Just remember that Eraqus believed that light was all and that darkness needs to be vanquished, he didn't believed in a balance.
Exactly why it's impossible to discuss with him. He's stubborn and close-minded, he wouldn't even consider the alternative Xehanort has to offer.
On a side note, Xehanort totally won to change mentalities. Because of his actions, Riku became a Keyblade user favoring balance between Light and Darkness. And Mickey said something like:
"I always considered Darkness evil, but seeing how Light and Darkness meddle in you, I'm changing my opinion" or something similar. Total win for Xehanort, Mickey or anyone else would never have said that by BBS.