Importance of matching in expo convertors?!

[jorgen.bergfors at idg.se]

>For a really low parts count expo converter, check John Simontons
>design for the Q control on the new state variable filter module.

>http://www.paia.com/syntfsch.gif

>If you stare at it long enough the temperature cancellation of the
>second transistor starts to make sense. This circuit works too.

What's the drawback with this circuit? There must be one, otherwise they would have used it for the frequency CV too.

/Jorgen