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 routable?
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. Stephen