Ya you don't have to leave the game. 

In fairness I have a personal policy. Once a game starts no matter how interested I am, or how far along the story is, I just don't join. I don't know why but all of my motivation gets shot once I start having to play even a small game of 'catch up'. So I if the situation was on me I would also be asking the Dm to hold off until I was finished. 

But I realize there are people not quite as picky as me who are fine with joining an in progress game. I'm just not one of them. That and some people post slower than others so that has to be taken into consideration.