Considering the census is used to determine how many congressional seats are allocated to each state, the question should absolutely be included. Illegal aliens or non-citizens, who are ineligible to vote, should not be counted toward elected Congressional representation in Washington D.C. In other words, they should not be counted to determine political power.
That is precisely the issue. Illegals should be, MUST be excluded from the census for the purpose of determining representation in Congress. People who shouldn't even be here (by law) have no claim to a voice in how our country is run, because if it was being run effectively according to our laws, they wouldn't even be here.
Sufficient illegals in some areas might conceivably give the illegals a congressional representative, representation equal to some of our less populous states in the House.