With infinite scrolling, do scrollbars still make sense?

I noticed this today on Google Plus:

Virtual Scrollbar in a Google+ Gallery

This is a virtual scrollbar, as opposed to native scrollbars everyone on Windows or pre-Lion OS X does know.

Now this happens to be a scrollbar in an infinite scroll context: that is, the actual document (the gallery) is much longer, containing about 3000 pictures.

Yet the scrollbar acts like a native scrollbar: its whole length represents how much was downloaded to the client, rather than how long the actual document is, and the scroll itself gets smaller as more parts of the document are downloaded, until it reaches a minimal size still comfortable to drag with a mouse pointer.

I guess I'm one of the last guys here who said "no thanks" to Lion and perhaps everyone else is used to these tricky scrollbars popping up everywhere, from smartphones to desktop interfaces, so perhaps I'm one of the last scrollbar user on the whole World.

Still, it brings me a question: are scrollbars useful still, esp. with infinite scroll / large data?

The original scrollbar concept was simple: you have a document, and you're viewing only a part of it:

With traditional scrollbars, the scroll tells the size of the viewed part compared to the whole as well as how far we've seeked from the beginning

But with the advent of infinite scrolling, we've got this:

In infinite scroll situations, the scrollbar has no direct relationship with the size of the document

So, we see the downloaded part of the document. While this might be a technical limitation of native scrollbars, we're talking about a virtual scrollbar here: this actually had to be programmed by hand, and most possibly UX-designed.

Historically, a word processor (like, excuse me, Word) didn't load the whole document: rather, it stored the geometrical size of all objects to be loaded, added them up, made a scrollbar, and loaded only the part which was viewed, or already viewed. (*)

So my main question is: is there a need for a scrollbar in 2012, if there is, how should it look like (including fallbacks) and if there is not, what should come instead? Why, why not?

Our context would be consumer community sites, so, photo galleries and newsfeeds (whatever we call the "Your friend XY did something on our site" pages) , target audience 30+-5, in possession of a smartphone but using a desktop computer for certain tasks.

(*) (For technical history on this, read Design Patterns from the Gang of Four: this is called the "Proxy" pattern

Update: to make it clear: I'd like to see explanations, patterns, wireframes. "It depends" is not an answer: of course it depends! It depends on what, and "for example if X", what do you recommend, "but if Y", what do you recommend, examples, ideas, mockups.

Infinite scrolling is abundant today, and scrollbars are here with us since about the 80s (perhaps it's a Xerox Star pattern). Yet I feel their current combination is meaningless, nonsense. Given that we have the computing power to implement our own scrollbars, how that virtual scrollbar should look like for infinite scrolling? Why?

(Even with 50 as bounty and 15 upvotes, noone has any alternatives?)