You might not be seeing what you think you're seeing, WRT everything going into the same truck. Lots of municipalities (e.g. mine) undertake resource extraction from commingled waste & recycling streams. It isn't the same everywhere, but sometimes it's more efficient to just put the entire waste stream through the same extraction process to recover recyclable materials. The separate receptacles at the client side are obviously redundant, but sometimes those remain from legacy processing arrangements.
I heard this as well. There is a recycling plant that you need to run anyway to separate the waste/rubbish so you might as well use it to get the plastics and cans out. I think there is an argument that while the machine can do it well avoiding the machine does have benefits.
This also doesn't work for paper or cardboard as it would get soiled from what I understand.
The real benefit of separate sources is those machines cannot handle things like plastic bags (they get caught in rollers and jam the machine), dirty diapers, and other such weird stuff. Once you get those out of the stream you may as well separate a few other things that are not recyclable so you need less of the expensive separation machines. However you always need sorting machines - people will make mistakes and so you need to verify everything really is recyclable.
