Yes, we should.
And this question and self answer seem to ignore the "why" of why a user's posts are deleted, and well, what I feel is the spirit of meta.
One of the slightly unique things about the way many Stack Exchange sites are moderated is while we have some fixed rules about content, topics and so on, the community does have a significant amount of input over how we do things. I for one often reference back to meta when explaining to a new user why something was done a certain way.
As such, an answer isn't just the opinion of the person asking the question. It's the opinion of everyone who voted, and commented. Some of these answers might inspire alternate answers and are important.
Just because a user has decided he no longer wishes to be part of the site, doesn't mean we don't care about what they've contributed to the community - especially if this is someone who has bothered to put in meaningful meta posts or has at least read them.
It might be different if it's a person who is entirely there to troll (or has devolved to trolling due to some frustration). Even then, it's better to have someone to judge the posts, and not the person.
By deleting any votes from someone who has chosen to delete his account, we lose context, maybe even change a decision that was made in the pas, and such. It would clearly be counterproductive.