This is the same kind of criticism that DC++ recieved
when it was introduced, with multihub support.
Multihub-search tools have been around for a long
while, and still, the DC concept lives.

but I am not again multihub connection (as long as the number of hub is reasonable).

I don't see the problem. If users want to stay in hubs
and share, chat and have special content hubs, they
will.

IMHO, a such kind of search is unnecessary. However, I am not against something quite similar but more useful. It would be great to have an unconnected protocol (UDP) that will allow someone to search for something or someone (it would be great to find where someone is).
But I am not crazy, this feature should require a kind of hub registration else anyone will be able to do search. Using this, it will be possible to build a generic search engine usable thru web (for example).

Earlier, someone suggests a hub should have an unique "key" that a client should give to another client to be able to download from it. With this tips, we can confirm both clients are on the hub and avoid a leaking of bandwidth out of the really connected to hub clients.

Eric