Why is Senator Kirk not yet former Senator Kirk?

SusanAnne Hiller has an excellent question:

The Senate has voted on three pieces of legislation today that required 60 votes–to raise the debt ceiling to $14.3 trillion, to reduce the deficit by establishing five-year discretionary spending caps, and Ben Bernanke’s confirmation–all of which interim Senator Paul Kirk (D-MA) has voted on. In addition, there have been other Senate votes since Scott Brown was elected as Massachusetts senator that Kirk cast a vote.

The main question here is: why is former Senator Kirk still voting on these legislative pieces? According to Senate rules and precedent, Kirk’s term expired last Tuesday upon the election of Scott Brown.

My suspicion is that, if the Senate tried to vote on a cloture motion which was going to be a 60-40 partisan vote, so that the vote actually mattered, the Republicans would make noise and fight it. None of the votes that they've taken in the past week-and-a-half have been such that it would have made any difference which way Senator MA-Jr. voted.

I think it's a bad precedent, but it hasn't made a difference in anything, so it isn't something that would be necessarily productive to squawk about.