Uncle Dave.
The circle of 5ths is a great tool for beginners to figure out chords. Unless you are doing soloing, scales aren't that important. For very basic comp, or using an arranger, or strumming a guitar, the circle of 5ths is a great tool!

Let me do another example, a more complex one:

Let's take "Angels" by Robbie Williams.
In the key of D major.

Last part of the chorus:

D A
She won't forsake me
C G A D
I'm loving angels instead

Let's analyse

Tonic Dominant
? Sub Dominant Dominant Tonic

Ok here we have a chord that doesn't fit into the pattern, what has happend?
Well the song changes key... C is not a note in the D major key that song is in. But lets look again a little closer. The thing that actually happend is: The song changes key. From D major, to C major, then back to D major again. Using this pattern

Tonic Dominant
Tonic(in C) Dominant Dominant(in D) Tonic(in D)

If we look even further, and substitute a chord from the D major key with the C, the chord that shares most notes with C is Em. Let's try that..

Tonic Dominant
Paralell Sub dominant, Sub dominant, Dominant Tonic.

That works to.. not as good as the C, but it works.

The point of this is not that every song correctly conforms into the circle of 5ths, but that you can essentially play most songs using it in this fashion.

Doc-Z