Maybe I'm just being 'old fashioned' but I still don't see a reason for glass panes being added. I think its just that they don't look hardly any different from the regular glass block, because I like iron bars and use them quite a bit.
I think I would like glass panes more if they had a separate texture associated with them so that I could either add a window type frame to them or maybe make them look like stained glass.
I like glass more, because
A. When playing in Survival, it's less resource costly (I think) and
B. It fills the whole area you put it, and I don't like it when I have holes.
Also, I agree that panes need a separate texture, but they do server a purpose. For example, my friend once used it as railing for his floating house, because he hated fences. And glass panes are more clear-looking.
I prefer blocks simply because glass panes are a pain (you see what I did there) when you're placing them near doors, and I just don't like the fact that they're quite out of shape with out blocks and won't place the way you want them to sometimes.