Hi there Winterstone,
I was wondering the same thing about the true advantages, but it is still kind of interesting nonetheless.
Im not sure how 'involved' i want to get with this circuit
but i can add a little more information for this particular circuit...
First, the exact expression for the frequency is given by:
w=sqrt(4*R2*R6-(Rx-R1)^2)/(2*C*R2*R6)
and you'll note the presence of R1 and Rx in this expression, and that's because they have a slight influence (although it will be small in most cases). You might also want to note that if Rx=R1 the expression dissolves into your expression for frequency:
f=1/(2*pi*C*sqrt(R2*R6))
However R1 can not be allowed to equal Rx. And the relationship between R1 and Rx brings us to the next item.
It looks like the exponential part of the response requires that Rx be greater than R1. If this is not upheld, then we end up with a damped exponential and of course that wont get us anywhere near the operation required for an oscillator. We need an increasing exponential where we take control of the damping via some forced means (such as diodes or clipping). This also means the equation for Q can be modified to remove the absolute value signs and reverse Rx and R1.
Also, to investigate component variation on the frequency we can use this equation which does not impose any restrictions on any of the values:
w=sqrt(4*C1*C5*R2*R3*R4*R6-C1^2*R2^2*Rx^2+2*C1^2*R1*R2*R3*Rx-C1^2*R1^2*R3^2)/(2*C1*C5*R2*R4*R6)
where
w=2*pi*f
I hope this information helps a little but as i said im not sure how involved i want to get with this circuit although it is interesting.
I do also have a question:
What parasitics are you most interested in, op amp internal gain and/or others?