On the other hand, Twitter clients which rely on polling seem to make their users happy. I see nothing in the spec about supporting polling, i.e. how a client might ask a hub for its version of a feed, but that seems to me like it might be a real useful function.
I don't think that adding polling to PubSubHubbub (PuSH) would be a good idea. Removing the need to poll from feeds is one of the reasons for its existence.
Also, polling is already possible using HTTP GET requests to the feed. So there is no need to add an extra layer between the server and the client, further increasing the complexity.
The only extra functionality that a Hub would add is caching, but that is not something we need Hubs for. There are already better ways to having caching in feeds (i.e. Feedburner).
PuSH was created to make realtime feeds a possibility. There is no need for extra layers of complexity that don't further that goal.