If the gaps are small, there are ways to fix it. For some reason I think I've heard that metallic nail polish works for it? That doesn't sound right now that I've typed it, but I'm positive that if the gaps are small it can be repaired. I'd research it and try that before getting into replacing glass.
In fact, if none of it is defrosting you might have a different problem. I think ordinarily when you get a break in the resistor grid, you just lose a section after the break because it's basically one big parallel circuit (with about a hundred legs) and each leg/line will work independently of the others.