The issue seems to me more conceptual than purely scientific.
FDSN as it is consists of a growing number of stations and with so
many stations it is no longer as manageable as it was some time ago.
E.g. if data stops flowing in from some particular station, it is no
longer feasible to look after this and inquire why - especially in
places like Europe where FDSN stations are numerous and one
can find another station from an alternate location fairly nearby.
So - it appears to me - there is this desire to have some stations
designated FDSN-backbone, so that these stations make a
more-or-less uniform global seismic network with some assured
level of data availibility (open to discussion if this availibility level
is to be 95, 98 or 99%).
This matter is not easy. On one hand I do not see a reason why
FDSN should not accept another station that is broadband, low
noise and offering free data. On the other hand I can imagine
someone putting up a dense broadband network on a small area
and offering all its data (e.g. 100 stations) to the FDSN. It is not
the point so that FDSN becomes a free data repository to just
anyone (who e.g. might wish to save money on disk array - keep
the data just on his PC and have a backup copy at the FDSN
for the just-in-case). The line must be drawn somewhere.
It should be the FDSN leaders to decide on this, but to my
opinion the FDSN-backbone network should be much denser
than the CTBTOs. Or the interest in the FDSN-gathered data
will be limited.
Of course one must keep in mind that the CTBTO network is
cunning - it has been constructed with maximum possible station
density that is still more-or-less uniform throughout the world.
So should FDSN go to a greater station density, then naturally
there would be regions where getting a greater station density
than CTBTO shall not be possible. Moreover, FDSN is unlikely
to surpass CTBTOs resolution in areas like Russia or China where
much of the data is restricted; according to the CTBTO treaty
the data is ported to the ISC, but digital data gets then restricted
there at Vienna.
It is a separate question if FDSN should archive all the data or
perhaps use some distributed-archive scheme. But with distributed
archive, the number of data contributors, and lack of funding for
specifically running the distributed-archive at the contributors'
sites - managing a distributed archive may be a problem.