[sdiy] voltage controlled filter???

[synthplayer88 at spymac.com]

Hi guys,
I was wondering if anyone can tell me whats the different betwen using
transconductance amp like the CA3080 and the LM13700 or the use of balanced
modulators/multipliers to control the cutoff frequency of say a state variable
filter, instead of dual rev log pot?

Is is because dual rev log pot are hard to find?

In order to use transconductance amp or multipliers don't you need to generate a
rev log voltage in order for it to work like a rev log pot?

I am guess people are using dual gang linear pot wiring it like this

http://home.comcast.net/%7Esbernardi/elec/og2/partsub_pots.html

to get the rev log voltage then control a pair of the transconductance amp or
multiplier with it?

Isn't it much straight forward to use a dual gang rotary switch  and work out the
resistance for cut off frequency?

I guess my question is, why do people use VCF in synth? why voltage controlled?
For modulation prupose?

Howcome I don't see people mention the use of balanced modulators, VCAs and
multipliers instead of the CA3080 and the LM13700?