I don't know about the specifics of ADS, but this question:
o does anyone see why the exchange address space needs to be globally
is valid in any case.
Global announcement of IX address space has caused me trouble in the
past when "remote" maintenance access depends on it; inconsistent
policy with respect to IX prefix announcement caused the path to the
IX prefix to be wildly lossy, while announcement of a non-IX prefix
was much better. We addressed that by making sure that all equipment
was maintained and monitored remotely using non-IX address space
addresses so that there was slightly better control over the
announcement (and over packet-level access to addresses on which
monitoring was permitted).
That said, I think it's still important to have global reachability
for exchange point addresses so that problems like congestion can be
remotely diagnosed. Sometimes the difference in packet loss between a
provider's IX address and some address one hop beyond it is a valuable
clue in tracking down problems; without global routing of exchange
point addresses, such diagnosis would only be possible to those who
distribute the IX prefix within their IGP.