That illegals will vote one way or another, that is.* A person need not be eligible to vote or even to register to vote to be counted on the census. Adult citizen, minor citizen, legal foreign resident, illegal foreign resident...all count towards the overall population on the census. And because it's as you say (the census being used for all kinds of purposes), that's as it should be.

Now, should that complete count be used to determine a state's allotment of Representatives? That's a very good question.

* and for the most part, they don't anyway: no assertion of widespread election fraud via ineligible voters has ever seen anything resembling rigorous proof