It's an exciting record because no-one's done it before, no one's got close and it's hard to imagine anyone matching it for decades to come. We generally don't add international hundreds, but that's because no one else has given us such a reason to.
And yes, ODI tons matter too, because they matter to him and to his fans. To use a PEWSian, it's his job to score not just Test hundreds, but international hundreds. And he's done it massively more so than the next guy.
Besides anomalies like Ramprakash, nobody counts the most First Class hundreds any more, because the game has changed to the point where domestic cricket is less relevant. But the enduring record of Jack Hobbs' career is his 199 FC tons, not his Test figures. We make an exception in Hobbs' case, because that number shows that he was just that good. We can do the same for Tendulkar.
I don't especially care about the specific mark of the hundredth hundred (and I'll certainly be hiding from Cricinfo for a while when he gets it - everything he does is a record these days, we don't need a bunch of new articles every time) but long after he has retired it's his international hundreds that should be the figure he'll be most proud of, and the one people should rave about, if they must.
It's his thing. It's what makes him special.