as i understand, the fm signal, due to its nature of changing rate of
phase change generates a number of sidebands. Filtering these
sidebands would mean that a band-pass filter is being applied to the
fm signal. That would amplitude modulate the signal as well. Amplitude
modualting would create some more sidebands but within the filter's
band-pass.
Finally, we would arrive at a 'least-bandwidth' signal that would
resemble SSB. So you might as well expend five crystals (for a ladder
filter and an oscillator) and get good ol SSB going.
A more intiutive example would be to consider an FM signal being
modulated by a single tone. That would waver the carrier back and
forth around the center frequency of the carrier. Now, if you passed
this through a band-pass filter, you will see the amplitude drop off
at the filter's skirts. This will resemble an amplitude modualted
signal. depending upon the filter bandwidth, you might see either an
AM, or a two-tone (carrier center being one, the modulated tone the
other) SSB signal.
I may be completely missing the point though, i welcome an
explanation.

- farhan