Fwiw, I don't think the census is completely useless.
From what I understand the original point was mostly to give those who would rather express their talossanity in some other way than voting an alternative. The downside of that is that perhaps fewer people vote, the upside is that perhaps more people who don't like politics stick around. I suspect both these effects are rather minor and only concern a small percentage of the population.
In addition, we get some nice statistics, and I was hoping, with Iasons help, to use location data from the census to track citizens who are assigned to the wrong province and provide them the option of changing province. None of this is crucial to our existence, but it's nice, and I think the census hasn't lived up to its full potential yet. (I had hoped to ask mpf at some point in the future to make it so that the SoS can send send out census emails themselves, and maybe change the questions, but I never got around to that.)
The thing about incorrect contact information though is that you will get neither the census nor the ballot when the chancery hasn't got your email and the latter will just always result in a lot more people (namely, politicians that want to be elected) telling you to do something about it. As a result, fixing email addresses has become pretty much a main part of running the election.
Throwing out citizens who vote but dont fill out the census would have a crippling effect I fear.
Compulsory voting on the other hand, I'm not so sure about. There is definitely a good argument for it.
However, perhaps the current system isn't all that bad. It's a bit messier maybe, but mostly it does its job at keeping Talossans on board, without filling the rolls with people who have permanently left Talossa behind.