Learning to scratch build model windows can expand possibilities for a model builder or hobbyist. You're no longer limited by what's available off the shelf.
Scratch building windows is really not that difficult. Here's a basic technique for double-hung window construction. Build a few and you can get pretty efficient at it.
Of course this basic technique can be expanded and added to. You could add more prototypical details like a sloped sill and additional trim and stops. Or even use those stops to make windows openable if you like.
Music tracks "Spanish Summer" and "Medium Rock" courtesy of Audionautix.com. Used under a Creative Commons license granted by the artist.