I can think of so many ways to justify > 5GB per month that I really can't list them all here, but here are just a few that I have personally done:
1. Offsite backups.
I've done frequent rsync backups of my cohosted webservers. This alone can take WAY more than 5GB. I've also backed up local files to offsite servers.
2. I work on robotic systems and have sometimes worked at home. This can involve working with HUGE amounts of recorded data (this could be something like four 60fps HD video streams, hyperspectral video, FLIR, RADAR, 3D LADAR, etc... and lossy compression is not acceptable). Typically, I'd only download a small sample data set to work on, but that could be easily WAY more than 5GB.
3. Photo uploads
I do photography as a hobby. Sometimes I get > 5GB of photos I want to upload at one time.
4. Web design
I do a bit of freelance web design on the side. Typically this involves modifying an existing site. I will typically set up a testing server at home and download the entire site to it, do my changes and testing there, then upload the final product. This can consume more than 5GB easily.
In practice I am currently averaging 160GB down, 80 up in a month.
This is beside the point though. If I had a 5GB cap, I'd probably not do these things. The fact that I don't is what allows me do do them.
If I had a 5GB cap, I might not even complain too much about it. After all, I wouldn't be doing anything that needed more bandwidth.
Sure, I don't need to use more than 5GB/month. I also don't need the internet at all. For that matter, I could probably just get rid of all my home computers since I don't really need them. I didn't always have them for that matter. Yet having them allows me to do things I could do otherwise.
My point is that deciding what people are allowed to use based on what they need right now is a bad idea. If we do that, we discourage additional development, we end up stuck with the internet as it is now. If we do that, in 20 years people will still be satisfied with their 5GB limits and they won't even know about all the great things that could have existed otherwise.
The excess bandwidth allows for innovation. The internet isn't just about E-Mail and web browsing. It is a general purpose communications network that can be used for whatever people need it for, including things that haven't even been thought of yet. That's what makes it so powerful.
