I came across this device just now, and I can't think of any reason for it to have the window that it has.
This isn't just a cutaway display image, either; the datasheet specifically mentions that you should avoid getting glue under it to avoid cracking the glass:
So, what is the purpose of this window? Surely it increases the cost of manufacture, so it wouldn't be there without some good purpose, right?
I can't see it being for trimming, either, as what seems to be exactly the same part is also offered in a different package that doesn't have a window.
@SteveHubbard has it right. The crystal has to be hermetically sealed preventing gas migration into the chamber, so plastic alone will not work. Glass is an excellent material for creating gas-tight hermetic seals with through-conductors. The crystal chamber will be filled with a dry inert gas or a vacuum.