Both sides on my case actually have windows on them. I've had this same case for quite a while so back a few years ago I thought it was cool
I was doing the whole "UV cold cathode" thing before case lighting was really popular.
If I was getting a new case though, I probably wouldn't get another window.
The only practical thing about it is that you can check things without opening the case (Make sure the CPU fan is still running etc)
Something that you might not know about it that dust shows up pretty easily on them and it's kind of hard to keep them looking good.
There are plenty of windowed / lighted cases out there that are good though, so it's totally up to you.