Possible to freeze jam in glass? I have read on multiple canning sites that you can freeze jam in glass jars. Has anyone actually done this successfully? Under what circumstances would it be preferable to putting the jam in plastic containers? 

It just doesn't seem very safe to me (I fear the glass would break).

Previously, I have bought plastic containers to freeze jam in, but they did not seal very well. I'm wondering if glass could perform better? Or, should I just find better plastic containers?
Yes, I've done this.  The key is to ensure that the jam isn't filling the glass container completely so that as it freezes it has room to expand.

I used a typical Ball canning jar, which sealed well enough to keep ice crystals out for several months.

From what I understand, freezer canning is useful when you don't have a pressure canner but want to can lower acid items which otherwise might be dangerous to your health (if not pressure canned).