MOTM

I think you answered your own question. Normal arrays of 2 pole 
bandpass filters would have a large amount of band overlap due to 
their rather gentle slopes . If you use an array of high Q filters, 
the bands would be so narrow that you'd have to use a zillion of them 
to cover the spectrum. A good compromise is to use a 2 pole filter in 
the center of the band, with a high Q filter at the upper and lower 
edge of each band to "square the corners" and maximize the separation.

Moe

>>>>>
Out of curiosity, what sort of response is this and why is it good for
vocoders? (I would guess that for vocoder channels you'd want plenty 
of discrimination between channels.)
<<<<<

>
> A typical BPF (like the Emu UAF, Oberheim SEM filter in BPF mode, 
> and many many similar filters) are two pole designs. For a good 
> vocoder filter (a single vocoder BPF!) you'd connect 3 such filters 
> in series, two of them with the same Q factor, the 3rd with a 
> different Q, and center frequencies spread such that the "middle" 
> filter is halfway between the "outer" filters on a log frequency
> scale. Two such blocks are needed for analysis and synthesis of one
> channel, and 20 channels were a good choice for the "classic" 
> Sennheiser vocoder.